There are verbs where the two are interchangeable, and there are verbs where they are not. Usually if they're interchangeable, when you use a pronominal suffix (like in this sentence), only אל can be used. So:
The dog runs to dad = הכלב רץ אל אבא/הכלב רץ לאבא
The dog runs to you = הכלב רץ אליך (NOT: הכלב רץ לך)
what radagast said is true, but i'll add that אל means generally 'towards' while ל means more 'for' and 'to'. In English, to and towards can be interchangable and thats the way it is in Hebrew too.