OpenType is the newest, greatest font format designed by
Adobe and
Microsoft, designed to supplant
TrueType and
Type 1 - it's cross-platform, to facilitate compatibility between platforms.
OpenType is an evolution of TrueType Open, and thus contains some of its components, such as its advanced glyph substitution and layout features. It also has a sort of digital watermark designed to prevent modification of the fonts.
OpenType's file structure is just like TrueType Open's: a series of tables indexed by a table directory.
OpenType's compression technology depends on whether its font information is Type 1 or Truetype; for TrueType, it uses Agfa's lossless MicroType Express. Type 1 uses the "Compressed Font Format" by Adobe.