A UFO file is a directory following this structure:

Each of the files has a unique meaning, purpose and data structure:

metainfo.plist Format version, creator, etc.
fontinfo.plist Various font info data.
groups.plist Glyph group definitions.
kerning.plist Kerning data.
lib.plist Arbitrary custom data.
glyphs A directory containing all glyphs in the font.
glyphs/contents.plist File name to glyph name mapping.
glyphs/*.glif A glyph definition.