Cheats in most games basically give extra help. In the case of Sims 2 they can also act as extra tools for the player, storyteller or builder.
Initialise cheats by opening the cheat box as explained below.
Most of the cheats listed on these pages are quite safe, so long as you remember to turn them off again. BUT you should make REGULAR BACKUPS of your game. The Skin cheat is strange and not well understood and I would recommend you use that one with caution.
The Time cheat will affect pregnancy and growth of Sim children and is safest used on a lot where there are only adults. Strange things happen: skeletons stretch, animations are off.
Houses made with Build Mode cheats will not affect your game if you download them - things put in place using these will stay where they are put.
To make a backup (which you should do regularly anyway!) just COPY the whole Sims 2 folder from your My Downloads\EA Games folder into another folder called e.g., Sims2backupJan12nocheats.
SeeThem by Dave Luv of Simsky is a very simple, user-friendly way to start with cheats.
Make a text file called userstartup.cheat in EA Games\The Sims\Config.
You can open this with Notepad, and put the cheats can be put in here instead of typing into the cheatbox. They will then will be activated when your game is started. These cheats can't be turned off while your game is running so be careful what you put in!