restart
restart is a console command available in all 
Source games.
The restart console command does exactly what it says. It restarts the map currently being played on. No stats will be preserved. If the map was transitioned to via a changelevel, the command will make the player spawn at a spawnpoint in the map (or the origin if there are none) and lose any weapons they may have.
reload
This console command will load the last saved state of the game. If the map being played on has no current saves, it will function like the map was restarted.
_restart
The _restart console command restarts the srcds binary. _restart does not, however, restart the graphical Source Dedicated Server, as it requires closing the window and reopening it in the Tools menu of Steam.
Note:When a client does this command, the game will close completely then usually start itself back up automatically.When given, this command waits 10 seconds before restarting the server. Many errors can occur that continue or cancel the restart. Segmentation faults are a prime example:
_restart PreMinidumpCallback: updating dump comment Looking up breakpad interfaces from steamclient Calling BreakpadMiniDumpSystemInit /srcds/css/css/srcds_run: line 374: 6909 Segmentation fault (core dumped) $HL_CMD cat: hlds.6901.pid: No such file or directory email debug.log to [email protected] Sat Jun 01 00:00:00 CDT 2012: Server restart in 10 seconds
This segmentation fault could have canceled the restart and simply quit the server, but the server continued.
Sometimes the server simply quits instead of restarting for no obvious reason:
_restart Reference Count for Material ___error (10) != 0 Reference Count for Material __depthwrite00 (1) != 0 Reference Count for Material __depthwrite01 (1) != 0 Reference Count for Material __depthwrite10 (1) != 0 Reference Count for Material __depthwrite11 (1) != 0 Reference Count for Material ___glintbuildmaterial (1) != 0 Reference Count for Material __particlesdepthwrite (1) != 0 Sat Jan 01 00:00:00 CDT 2012: Server Quit