Learn Russian: Слева магазин, а справа аптека. - On the left is a shop, and on the right a pharmacy.

In Russian, а is used to show contrast or switch focus from one thing to another. Since we are pointing out one location and then switching our focus to the other, а is the perfect fit. The word и is used just to link similar things together without a contrast.

Where is the word for "is" in this sentence?

In the present tense, Russian usually drops the verb "to be" (is, am, are). You just put the location and the item next to each other. So, Слева магазин simply means "On the left [is a] shop."

I've seen налево and направо before. Why use слева and справа here?

слева and справа are used for location, meaning "on the left" or "on the right." You use them to say where something is already sitting. налево and направо are used for direction, meaning "to the left" or "to the right," which you would use when telling someone where to walk or turn.

Do I always need a comma before the word а?

Yes! A strict rule in Russian punctuation is that you must always put a comma before the conjunctions а (which means "and" or "but" for contrasting) and но (which means "but").