Understanding the difference between perfective and imperfective verbs (verbal aspect) is key to communicating clearly in Russian and misunderstandings can arise for the uninitiated.
Almost every Russian verb comes as a pair: one contains a prefix and one does not. The two verbs clarify whether a process is ongoing or habitual with no definite end, or the extent to which an action has been or will be completed, a concept called verbal aspect. Verbal aspect is to the Russian language, what the tense system is to the English language. Whilst both Russian and English have only three main tenses (past, present, future), English goes further in containing four different types of each which a learner has to learn if they wish to have any hope of understanding and speaking English as well as a native speaker.
Mastering the tense system in Russian is as simple as memorising three relatively straightforward conjugation patterns (the change to the endings for each grammatical person): two almost identical sets for the 6 grammatical persons in the basic tense and one set for the 3 grammatical persons in the past tense. Take a look at the cheat sheet on that here.
As you can see, the two forms are used to convey different shades of meaning and are arguably more important than the tense system in Russian. There is no rule as to the precise prefix you may see before an imperfective verb in Russian and sometimes, the verb is completely different in the imperfective and perfective forms. However, there are some fairly common prefixes (for example, по-, про-, на-, за-, etc.) and I can promise you that once you start to recognise a good number of verbs in Russian, you’ll quickly begin to identify the fact that a particular verb is perfective (i.e. contains a prefix), especially because the same ones appear time and time again.
To clear up any confusion over verbal aspect, simply take a look at the clear explanations contained in the videos below. The first video contains a very clear and concise explanation of the difference, whilst the second much longer video takes you through a series of examples which will allow you to see the theory applied in context and hopefully make it much easier to master the concept.