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 1.8 versions
(includes incompatibilities with earlier versions) see:
- http://wiki.lazarus.freepascal.org/Lazarus_1.8_fixes_branch

* For an overview what changed from previous 1.6 versions 
(includes incompatibilities with earlier versions) see:
- http://wiki.freepascal.org/Lazarus_1.8.0_release_notes
- http://wiki.lazarus.freepascal.org/User_Changes_3.0.4
- http://wiki.freepascal.org/FPC_New_Features_3.0

* Checksums can be found at http://www.lazarus-ide.org/index.php?page=checksums

* For additional information visit our homepage, forum, wiki, and FAQ:
-- http://www.lazarus-ide.org/
-- http://forum.lazarus.freepascal.org/
-- http://wiki.lazarus.freepascal.org/Lazarus_Faq