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:
For an overview what changed from previous 1.0 Versions
(includes incompatibilities with earlier versions)