Brown algae is usually seen as an easily wiped off brown-yellowish slime which appears on all surfaces but most notably on the aquarium glass.
In many cases, if brown algae is left a green slime algae will grow over the top. Brown algae is common in new tanks or aquariums a few months old, probably because of the excess nutrients found in new systems (when tap water is used) and fluctuating water conditions.
Established tanks with growing plants experience very little brown algae since growing plants will compete for nutrients, however it is normal to have some brown algae, as well as other types, at a manageable level.
Brown slime algae can be easily controlled through the use of algae eating fishes, good water conditions, and fortnightly glass wiping.
Whilst strong lighting is often blamed for strong algae growth, this is not the cause of the problem and healthy, established tanks with strong lighting often have less algae problems than those with lower light levels.