Good vs bad bishops

How do you determine whether a bishop is good or bad? For example, if in my next move I can choose whether to exchange bishops or to save my bishop, what kinds of things do I have to take into consideration to make the best decision?

I guess that's not specific to bishops... How do you decide whether or not to exchange pieces of equal value?

This puzzled me when I first started playing. As I understand it, a "good bishop" has mobility and is not restricted by its own pawn structure. A "bad bishop" has limited mobility because the player's own pawns are on its color and restrcting its movement. Active = "good"  Locked = "bad"

Thanks for your reply, 1pawndown. That makes sense. But then how do you know wether or not to exchange other pieces? Like knights for example

A good bishop can be active or passive and this also applies to a bad bishop. A bad bishop is one in which pawns of its own color greatly limit its mobility ( most importantly center pawns ) . A great example is the pawn formation in the advance french , after 3 e5 both players have a good , and bad, bishop. 

Nimzovich's classic work, My System, has a chapter devoted to just this question/problem. 

Don't take the terms good and bad bishops too literally.  The terms describe the bishops relationship to the center pawn formation.   "Bad" bishops often guard good pawns.   Bad Bishops can sometimes become very powerful with a timely sacrifice of a pawn,  This often happens with isolated queens pawns, when the pawn suddenly moves forward. 

Thanks for all the useful posts and the resources, everyone! I understand a bit better. It's still complicated to me and I'm not sure I would be able to recognize black/white square weaknesses in my own games, but hopefully with practice and by studying I'll get better at it eventually...

Maybe this comparison will help bring the concept home:

Atos = good Bishop

Fezzik = bad Bishop