When playing Coutner Strike, the ~ key is 99% of the time bound to access the "console". This console allows players and server admins to change the properties of the game while playing, such as setting the maximum round time, the amount of money to start off with (default is $800), and even reduce the quality of sprites (eg. sprays, smoke grenade, etc.). Obviouslly there are many commands that only the admins can use (gravity level, time limits) and they are listed below amongst others. Also: x=0 means off and x=1 means on.

Command Name	Meaning	Client/Admin/Both
hud_fastswitch x	Allows you to make the weapon automatically appear without having to go through the "pictures" (sprites) at the top of the screen; Allows you to quickly access another weapon/item.	Both
mp_decals x	Allows you to specify how many bullet holes and spray paints can remain on the walls and floors of the level. Default is 300.	Both
fastsprites x	Lets you choose the quality of sprites in the game (eg. smoke grenade). If x=0, then normal sprites. If x=1, then there are moderately detailed sprites. If x=2, then there are very undetailed sprites (the lower the quality, the better performance you'll receive).	Both
max_shells x	Specifies the maximum number of empty bullet shells that will leave your gun.	Both
max_smokepuffs x	Specifies the maximum number of smoke puffs that appear where your bullets hit the wall.	Both
adjust_crosshair	When entered in the console, your crosshair color will change. Continue to enter this command and it will cycle through several colors.	Both
timeleft	When entered in the console, it will show you how much time is left in the game.	Both
cl_corpsestay x	Lets you specify for how many seconds the dead bodies will remain on the ground until they disappear.	Both
sv_gravity x	Allows admins to alter the strength of the gravity on items and players. Default is 800.	Admins
mp_startmoney x	Allows admins to alter the amount of money each player starts off with. The default is $800 and maximum is $16,000.	Admins
mp_buytime x	Allows admins to alter the amount of time that each client has to purchase their items from the store.	Admins
mp_roundtime x	Allows admins to alter the amount of time that each round lasts. Default is 9.59 minutes.	Admins
mp_timelimit x	Allows admins to alter the amount of time that each game lasts. Default is 45 minutes.	Admins
mp_maxrounds x	Allows admins to alter the amount of rounds that will be allowed to be played until the map changes.	Admins
mp_autoteambalance x	Auto Team Balance makes sure that both Ts and CTs have the same number of people (if an even number). Default is 1.	Admins
mp_c4timer x	Allows admins to alter the number of seconds the C4 needs until it explodes.	Admins
mp_friendlyfire x	Allows admins to alter the ability for the members of one team to be able to harm their own/kill team members. Deafault is 0.	Admins
mp_tkpunish x	If enabled, the person who killed a team-mate will sit out the next round.	Admins