There are two different kinds of algae you may experience in your fish tank, one kind being largely beneficial (unless the tank is also heavily planted) and the other kind a type that you will wish to dispose of.
The "friendly" kind helps your tank in much the same way plants do, they reduce the level of harmful chemicals in the water by contributing to the nitrogen cycle.
Some species can also provide food for algae-eating varieties of fish.
Slime-Smear Algae This kind of algae is generally indicative of poor water quantity and will spread over almost anything in the tank or aquarium.
An additional characteristic of this kind of algae is that it will also smother and kill plants.
This is actually cyanobacteria and although it is possible to physically remove it, this will generally not solve the problem.
It is possible to treat the water with 20 mg of erythromycin phosphate per gallon of water.
If this treatment is considered nitrate levels need to be closely monitored.
Brown This kind of algae forms soft brown patches and is generally indicative of a lack of light or high levels of silicates.
Better lighting and physically removing this kind of algae from the tank are both effective.
"Green Water" Unicelluar algae that bloom so quickly that the water of the aquarium rapidly turns green.
Special filtration cartridges are capable of removing this kind of algae, ultraviolet sterilisation will completely prevent it.
Spot Thin and round bright green spots that can form anywhere in the tank, spot algae can simply be removed with a bit of elbow grease.
Under strong lights this kind of algae may also occur on plants.
Fuzz Grows in short strands of 2-3mm, and is considered a kind of Beard Algae.
Many algae eaters are effective at consuming this form of algae.
Beard Begins by growing on plants and whilst the individual strands are very thing, its grows very quickly into a "beard".
Considered a negative kind of algae as it is impossible to remove without treatment (Simazine) and is detrimental to the plants it grows on.
Hair An unpopular kind of algae in America which is normally found in matted clumps growing from gravel.
Up to 5 cm long, this is normally easily removed with a toothbrush.
Many european fish keepers use it as a food supplement for algae eaters.
May be troublesome if left completely alone.
Staghorn Shares some of the characteristics of Hair Algae, only it is green-gray and grows in much longer strands.
Difficult to remove manually and often grows on aquarium equipment.
Brush Grows in small black patches between 2-3mm.
Although it looks black it is actually a kind of red algae, which needs to grow on slow growing plant leaves and aquarium equipment.
Siamese Algae-Eaters can be effective at controlling this kind of Algae.
Not all kinds are bad, but all kinds should be kept in check as they may have an adverse affect on your plant life which offers more benefits to your fish.