rounded and beveled corners

If you only want to have rounded corners on a path, you can use Effect → Stylize → Round Corners. This will apply it as an appearance, so it can be changed later easily. If you want to make the change permanent, you can go to Object → Expand Appearance.

Another method for rounded corners is to use offset paths. While this change isn't easily reversible, it does allow for beveled (chamfered) corners as well.

1. select a path
2. go to Object → Path → Offset Path…
3. key in a negative amount for your radius, then select Miter and click OK
4. with this new path selected, go to Object → Path → Offset Path…
5. key in the positive amount for your radius, then select either Round or Bevel
6. delete the original and first offset paths

Same as the Object Offset Path method, but you go to Effect → Stylize → Offset Path instead. When you offset the second time, you need to tell Illustrator to apply a new effect. The advantage to this method is you can tweak it later, but the disadvantage is it must be applied to the entire path whereas the Object Offset Path method allows you to use pathfinder tools to adjust other parts of the path as desired.