Garry's Mod is a sandbox game.

You have a wide variety of props from various Source games (depending on which you own), and multiple tools at your disposal. With these, you can build different contraptions, make vehicles, construct buildings, etc. As well as all the content included in the game, you can find a ton of mods online via the game's website or Steam Workshop. You can also make/script your own.

But the fun doesn't stop there. Multiplayer is a totally different thing entirely. You can find sandbox servers and build with up to hundreds of other players, or you could dive into one of the many user created game modes. From Hide and Seek to Role Playing, there is a huge variety of unique modes to play.

A few I would recommend are:

Trouble in Terrorist Town.
A group of 'innocent' terrorists must work with the detective(s) to discover who the traitors are and kill them. The traitors must take out the innocents, preferably without alerting anyone else. No one knows who is innocent and who is not, making for a sometimes tense experience.

Morbus.
A group of humans must hold out for a certain amount of time against aliens, or kill them all. There are two types of aliens: Brood aliens, and Swarm aliens. The Brood aliens get upgrades, and can transform into humans. Allowing them to get into groups and pick off people one by one. The Swarm aliens are what players spawn as when they die (assuming they were killed by another Swarm alien, or human). These have a melee and ranged attack, but no upgrades. Their role is to weaken the humans, and try to kill them.
The humans have needs. Every now and then you will have to either sleep, eat, shower, or pee. You cannot move during this time, so must be wary of those around you, in case they are brood aliens.
Overall one of the best game modes. A ton of fun, especially with friends.

DarkRP.
DarkRP is one of the more casual RP modes in Gmod. You spawn into the map as a citizen, and can choose a job to play as. From drug dealers to civil protection, there is a wide variety of roles to pick. You can go around earning money, commit crimes, buy and furnish your own house, etc. Great if you find the more serious RP servers to be boring.

If you plan to buy the game, be sure you own Counter-Strike: Source too. A lot of maps use textures from CS:S, and if you do not own it you will see purple and black squares everywhere.

Overall the game is absolutely brilliant. One of the very few that I can keep going back to and not get bored.

One of the best purchases I have ever made.