The verb пойтќ (irregular past пошёл, пошлђ, пошлќ) is Perfective and the verb ходќть (regular past ходќл, ходќла, ходќли) is Imperfective. The usage of these two verbs is roughly like that of взїть/брђть. Use пошёл when you want to specify that the action has an end point.
The end point of someone's 'going' (in the sense of 'setting out, taking off') is that he is no longer here. Thus, Ћн пошёл в библиотљку 'He took off for the library' is roughly synonymous with Егћ нљт 'He's not here', just as Ћн взїл кнќгу 'He took the book' implies Кнќги нљт 'The book's not here'.
The sentence Ћн пошёл в библиотљку doesn't say whether or not he got to the library, but we do know that he is gone. However, the sentence Ћн ходќл в библиотљку implies that he got there and returned.