The typical wear items are both made of rubber.
If it's leaking out of the top it's probably the packing washer.
Dripping out the bottom probably the stem washer.
Both can be replaced without too much work but in some cases replacing the entire hose bib might be the best option.
No hard and fast rule - every situation is different.
I bought a kit of replacement washers and keep it on hand and many times can do the repair without going to the store.
This is an example of a kit:
And a diagram showing the two different washers:
