Originally Posted by chaika
Dogboy quoted you probably the only verb in the Russian language where the prefix по- does not function to create a perfective verb. At least I can't think of any others, but maybe he can. Otherwise по- added to a verb makes it perfective. Sometimes it adds no additional nuances, just adds "perfectiveness", but often it can add the notion of "a bit" as noted above. Also means the beginning of an action, as in пойти "set out," from идти "walk". In general, adding a prefix to any verb makes the verb perfective and adds an additional nuance to the meaning. When you learn verbs, you usually learn the imperfective form PLUS the corresponding perfective form with no additional nuance.
To find out the skinny on questions like this, you need to consult A Comprehensive Russian Grammar by Terence Wade. This is the kind of book you consult after your 4th year of Russian language study.
I wrote my dissertation on verbs,
чайка