Cheat codes are entered in a fill-in that appears in the game screen's top left corner when Shift-Control-C is pressed on the PC, and Shift-Apple-C on the Mac. On some wonky keyboards (like mine) the Shift-Control keys have to be pressed on the opposite side from where the C key is, or nothing happens; so if no fill-in appears, try keys on different sides.
There are several lists of cheat codes on the 'net. Many don't work. Some (like the water cheat) have been made obsolete, for instance, by Maxis adding a Water Tool to Build Mode. The famous money cheat (remember: "rosebud" now, not "klapaucius") works, as does the infamous move_objects cheat.
As is mentioned in several places, a cheat can be repeated with "!", so a quick way of gathering money is "rosebud;!;!;!;!;!" etc. Ctrl-C and Ctrl-V work in this box. That also means I could accidentally paste an URL or a letter into it. Not to worry; the game will simply protest "No such cheat!" and ignore it.
(Incidentally, to get money, I can do other things, which are also found as tips on the web: backup the house IFF, sell everything in the house and break it down - there's money in old walls! - and then overwrite the stripped-down lot with the backup, clicking on each family member to make them visible again. (Don't do this if the family has pets, they may become strays.) Or I can have a Sim launch the rocket, wait until it starts coming down, switch to Buy mode and then select the rocket, or explosion depending on how fast I reacted, and "sell" it for 2300 Simoleans. There is another good reason for catching and deleting the rocket as it comes down - it can land on, set fire to and kill a passing Sim! And Sims are always attracted to rocket launchings.)
The move_objects cheat is best NEVER used in Live mode. Not only is it possible to delete vitals like the pedestrian portal, the carpool or the mailbox, but deleting a Sim (a popular means of restoring all the Sim's needs; click on the icon and the Sim reappears, totally refreshed) can destroy pending social interactions. In my case, a neighbour's visit was deleted. I could now no longer phone that neighbour, nor did the neighbour ever turn up. And the Relationship points kept dropping. The only solution was to replace the house IFF file of the afflicted family with a backup. There are safer ways to up a Sim's needs!
In all, I've found two applications for the move_objects cheat: in Build mode, to build outside the lot's boundaries (or even on them - normally, a wall has to be at least one square away from the lot's edges) and to rescue a cat who couldn't get down from climbing apparatus; I had to delete and restore the poor cat, who was tired and starved and mewing loudly, and "sell" the apparatus, which couldn't be moved or deleted normally as it was "in use".
The move_objects cheat (type "move_objects on", and "move_objects off" once finished) allows building outside the lot's building grid, and lets the user place things outside this grid while the game is running. There is also a lot_size cheat to increase the size of the lot's building grid. I've never used this one myself, but some fan-made houses up for download use all the space available on the lot, and it would be fun to, for instance, have a barbecue in the middle of the road - though the carpool won't be able to leave if something or someone is blocking the vehicle! The Sims themselves are not hindered by the grid's boundaries, and happily swarm all over the area.
A cheat that I have used myself is map_edit. Type "map_edit on" and click on the grid squares to (de)activate them. Rather than increasing the rectangle on which I can build, this makes specific squares accessible or non-accessible to the player. The area that is made accessible also shows up on the neighbourhood screen, so I could put a picnic table on the middle of the road and it would be shown in the neighbourhood. I could also make squares inaccessible within the house, even if they've been built on or have had objects put on them, for a kind of ghost zone: I wouldn't be able to click on the objects there, or move them in Buy mode, though the Sims can still use them. This is another cheat that should be turned off when no longer needed. I've found I had to turn it off before leaving a lot and going to the next one, or the cheat wouldn't work on the next one.
All ten lots made completely accessible with map_edit; they no longer
blend into the background, and some edges overlap!
Warning: do not bulldoze a lot that has been made completely accessible
with the map_edit cheat. This cheat makes all the editable squares visible in the
neighbourhood screen, so you can finally see those roads and sidewalks. But it also
makes them bulldozable, and the bulldozer will remove all the floor tiles (that's
what the road and sidewalks are made of) and that, too, is instantly visible on
the neighbourhood screen. Instead, replace the lot with the original empty lot
(house files can be changed while the game is running; switching to another
neighbourhood and back will load them) or just break it down item by item. Unless
you want the neighbourhood screen to be bare...