Can I put 18 inch rims instead of 17 inches?

If the wheels are the correct bolt pattern and offset you can use them. You will need 45 series low profile tires which (in my opinion) is not a good way to go for snow tires.

What's an offset? I just know that both have 5x120 bolts.

You might want to look that up on the web as I can't post a link but offset is how the wheels are centered on the brake rotor. Some wheels stick out farther than others. With the wrong offset you can have clearance problems. wheel bearing or handling issues or the tires could stick out too far.