What Shiny said. It's like a giant physics sandbox. Alone it's like Minecraft classic.
But also like Minecraft classic there are tons of different nodded game modes. Detective games (Trouble in Terrorist Town), Roleplay (tons of variations), Zombie survival, and more.
And singleplayer is just as great, since you can get tons of stuff if connected to the net. You can create animations, or download hundreds of new toys. My personal favorite is using the Orbital Raincannon (I believe it's called). It's quite, err... Epic. Monsters from fantasies can come to life as you bring in Weeping Angels and more. Or, create a giant rollercoaster.
The goal of GMod? It's the same as Minecraft. There is none, except to have fun.