These are the Lazarus sources and help files. You need to download and install FPC for your platform to build 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 1.6 versions
(includes incompatibilities with earlier versions) see:
- http://wiki.lazarus.freepascal.org/Lazarus_1.6_fixes_branch

For an overview what changed from previous 1.4 versions 
(includes incompatibilities with earlier versions) see:
- http://wiki.freepascal.org/Lazarus_1.6.0_release_notes
- http://wiki.lazarus.freepascal.org/User_Changes_3.0.0