Verbs of motion (or verbs of movement) is one of the most complicated topics in Russian language. We all remember that there are two non-prefix types (unidirectional, like идти, ехать, бежать, лететь; and multidirectional, like ходить, ездить, бегать, летать).
But when should we use this form and when that form? That is always a question. In terms of Russian and English languages comparision we might say (it is not 100% correct, but it explains the situation quite well): unidirectional verbs must be translated with continuos tenses (Я иду / я бегу = I am going / I am running; я шёл / я бежала = I was walking / I was running); whereas multidirectional verbs are translated with simple and perfect form (я ездил в Москву = I've been to Moscow; я часто езжу в Москву = I go to Moscow quite often).
Here you can find the worksheet about "ехать - ездить" pair of verbs and audio file! Have a nice day!