What is the difference between unidirectional and multidirectional verbs of motion and when should I use them?

Verbs of motion are one of the most complex aspects of Russian grammar and can be very difficult to get your head around. 

First and foremost, it is essential to remember that whilst most verbs in Russian come in imperfective/perfective pairs, unprefixed verbs of motion such as ходить/идти are pairs of multidirectional and unidirectional verbs, and only become imperfective/perfective when prefixes are added.

Multidirectional verbs such as ходить are used to indicate:

1. General or repeated action, e.g. going somewhere regularly. Я ходила в школу каждый день - I went to school every day (for a number of years)

2. Return trips, i.e. there and back again. Мальчик ходил в кино (and came back again) 

Tip: Look out for adverbs and adverbial phrases such as часто, редко, каждый день, никогда, обычно, etc. - these indicate that the multidirectional verb of motion should be used.

Unidirectional verbs are used to indicate:

1. A single trip in one direction, e.g. Иван идёт в магазин - Ivan is going to the shop (and has not yet come back).

2. Setting out to go somewhere/declaring the intention to go somewhere e.g. Завтра я еду в Ростов - Tomorrow I will go (by transport) to Rostov.

Here the action of travelling to Rostov is emphasised, and not the duration of the stay or details of the return, so the unidirectional verb is used.

When prefixes such as по- or про- are added, the unidirectional verb becomes perfective (denoting a singular, completed action in the past or a future action) and the multidirectional becomes imperfective (denoting a repeated action in the past, present or future.)