It's a prime example of how tenses are used in Russian as compared to English. I don't know the name of this phenomenon, but /u/KweB's explanation (this comment) is wrong.

In English this difference is expressed by Simple vs. Continuous tenses. For example, when you're flying somewhere right now, it's Continuous, and when you fly somewhere regularly, it's Simple. Russian has only three tenses of verbs - past, present and future - without all these "modificators" (simple, continuous, perfect) - instead it uses different verbs to convey the same idea, which is exactly what you are asking about.

Бегать is "to run" in the Simple tense. "Я каждое утро бегаю" - "I run every morning" (in the meaning of exercise). Бежать is "to be running" in the Continuous tense. "Я сейчас бегу" - "I'm running right now" (so call me later).

Russian language has a lot of these pairs of words, and they're all verbs of motion. "Лететь" ("to be flying") vs. "летать" ("to fly"). "Идти" ("to be walking") vs. "ходить" ("to walk"). The explanation of unidirectional vs. multidirectional that the other user provided doesn't hold for all situations, although, admittedly, it can work in some cases. If you're talking about walking somewhere, the choice between "идти" and "ходить" depends entirely on whether it's "...to be walking (right now)" or "to walk (often)", not on the specificness (is this a word?) of the place where you're going.

Verbs that are not verbs of motion typically don't have two pairs. For example, "звонить" (to call on the phone) is the same for both Continuous and Simple situations. "Я сейчас звоню ему" - "I'm calling him right now", "Я звоню ему каждый день" - "I call him every day".