In his Physics, Aristotle defines four "causes" to set the background for his explorations into nature. They are formal cause, efficient cause, final cause, and material cause, and all four can be related to every object, whether it be natural or artificial. After first defining each cause, Aristotle adds that the former three "often coincide" -- that is, that the properties of a given object that each cause defines sometimes overlap (Physics book II, pt. 7). Despite the overlap, each cause is still extant on its own, but because they cover the same area they need no longer be considered individually. Proof of and support for this proposition lies in the assumption that "nature is for the sake of something"; this allows each of the causes to become knowable, particularly the formal and final causes, similar though they are (II, 7). As with much of Aristotle's work, the end goal of his proposal that some causes can be considered as one is to ease and facilitate our understanding of the natural world.
In order to deal adequately with its parts, and in this case its constituent causes, nature itself needs a suitable definition. For Aristotle, it is that which "has within itself a principle of motion"; that is, nature is constituted of all those things which in themselves are capable of changing in one way or another, as opposed to artificial things, which are not possessed of this "innate impulse to change" (II, 1). This is relevant to the problem of coinciding causes because of its relationship to and effect on efficient cause, one of the three that overlap. In nature, efficient cause (the particulars of which will be dealt with shortly) comes from within; where artificial things are concerned it comes from without, and thus this is one of the ways in which the natural might be distinguished from the artificial. Another critical aspect of "nature" to consider is that it is comprised of two parts: "the matter and the form", where all facets of an object exist "for the sake of" the form, including the matter itself (II, 8).
The four causes are also constituent parts of nature, as they are of all things. Material cause falls outside the realm of interest in this case, because Aristotle does not group it together with the other three; this leaves only formal, final, and efficient cause to be considered. The first of these (formal cause) is an object's "form or [its] archetype" (II, 3). This concept is similar in principle to Plato's doctrine of the forms, but instead of being external and on a higher plane apart from the world of the sensible, Aristotle's "forms" are contained within the objects or creatures themselves, like a sort of intrinsic logic that directs the shape that an object takes. Aristotle exemplifies this with the mathematical image of a straight line, the formal cause of which is, simply, "line"; in the more general terms possible, its formal cause is its definition, or that which answers the question "What?" when it is asked in relation to any object (II, 7).
A second cause is final cause; something's purpose, or "that for the sake of which" an object or creature exists, or in some cases, the reason why a particular action is performed (II, 3). This cause is not only associated with a singular thing or action, but also with all of the steps in between that serve as "means towards the end"; as examples, Aristotle lists off various ways in which one might aspire to health -- by walking, or through surgery or other means of "reduction of flesh" -- and despite their disparity, each one is motivated by the same final cause and thus they can be thought of in the same way (II, 3). Everything, be it an object or an action, exists for and proceeds toward its final cause, despite the fact that "they differ from one another in that some are activities, others instruments" (II, 3).
Aristotle goes on to say that in many cases, formal cause (the "what", or definition of an object) and final cause ("that for the sake of which" it exists the way that it does) are the same thing (II, 7). Insofar as objects and creatures go, this is a fairly intuitive conclusion, for from observations it looks as though natural things exist in the forms that they do for a reason; and in typically abstruse and roundabout fashion, Aristotle exemplifies it most appropriately and effectively by bringing into play its opposite: the "man-faced ox-progeny" that the pre-Socratic philosopher Empedocles thought to exist, which passed away because they were not formed such that it seemed that "they had come be for an end" (II, 8). It follows logically that if something exists in a form (formal cause) that is suited to its purpose (final cause), formal and final cause are one and the same, and Aristotle's claim is correct.
The third of the causes that might be grouped together is efficient cause, defined as "the primary source of the change or coming to rest" of a given object (II, 3). Whereas in artificial objects the efficient cause is external, stemming from a mover outside the object, natural things contain efficient cause within their very matter (II, 1). In the broadest of terms, says Aristotle, efficient cause is "what makes of what is made and what causes change of what is changed" - that is, whatever it is that is responsible for a change in either itself or in something else (II, 3). One of the first examples given of efficient cause is that "the father is [the] cause of the child" (II, 3). From this it might be interpolated that the efficient cause of any creature is another creature, of its same kind.
It is this assumptive characteristic that allows Aristotle to place efficient cause with formal and final cause. Because "man generates man", the "primary source of motion" is identical to the form which it producing; and so it is with all creatures, or "all things which cause movement by being themselves moved" (II, 7). An example that might be elucidatory here is that of an oak tree growing from an acorn -- its formal cause is the shape of the tree into which it is growing, according to its kind; its final cause is that same tree, proving that in this case these two causes are identical; and, according to the idea of man begetting man, its efficient cause is the oak tree which produced the seed from which it is growing (for "what grows qua growing grows from something into something" else, as its purpose is defined by formal and final cause) (II, 1). Thus, the three causes can share a single definition.
The last remaining cause, which Aristotle leaves on its own apart from the others, is material cause -- the very stuff out of which a given thing is made, or "that out of which a thing comes to be and which persists" (II, 3). The reason that it is not considered as one of those causes that coincide relates back to Aristotle's definition of nature as comprising both matter and form, as cited above. Matter itself is not an active participant in the procession toward final cause; instead it falls into place as being merely "for the sake of the end" that is defined by form (II, 8). It is not definitive of the change that occurs, though it "persists" for the duration (II, 3). Because of this, Aristotle cannot group material cause with formal, efficient, and final causes, and so leaves it to be considered separately.
Arranging the four causes in this way is useful for the natural philosopher, because it allows one to distinguish between that which is "inside the province of physics" and that which is not, based on whether or not formal, final, and efficient cause coincide (II, 7). Those things that "are not of this kind" are identifiable because "they cause motion not by possessing motion or a source of motion in themselves" but instead cause motion whilst at the same time being themselves immobile (II, 7). This means that if the efficient cause of a given object cannot be associated with formal and final cause, regardless of its material cause, it cannot be thought of in the same way as those things whose causes coincide; that is, that it is outside physics, and therefore must be thought of in a different way -- and the ultimate "unmoved mover" is Aristotle's god, who is utterly transcendent of the physical world which we inhabit.
It is only in understanding important distinctions such as this one that one might come to greater knowledge of the natural world and those things contained within it -- which, of course, was Aristotle's goal from the outset.
All quotations above come from the translation of the Physics available at http://classics.mit.edu/Aristotle/physics.html.