What causes leaves to turn brown?

If you mean the whole leaf getting brown, they can be caused for a number of reasons.
An infestation of pest, dry soil, low humidity, high heat, too much direct sun.

If it's just the tips. Some plants are susceptible to brown tips such as the peace lily, spider plant and a few more. Causes can be from using hard-water use (use distilled or rainwater when possible) also, over-watered, or under-watered, low humidity, heat, too much fertilizer, or any combination.