You need to download and install all three packages fpc, fpc-src and lazarus.

fpc - the Compiler, some command line tools, base units and non visual components like database access
fpcsrc - the sources of fpc and its packages, needed for code browsing
lazarus - the IDE, visual components and help files

For an overview what changed from previous 1.2 Version
(includes incompatibilities with earlier versions) see:
- http://wiki.lazarus.freepascal.org/Lazarus_1.2_fixes_branch
- http://wiki.lazarus.freepascal.org/User_Changes_2.6.4

For an overview what changed from previous 1.0 Versions 
(includes incompatibilities with earlier versions)
see: http://wiki.freepascal.org/Lazarus_1.2.0_release_notes