You've seen the slimy green residue that sometimes creeps up the sides of your pool and know it's algae; thousands of one-celled plants that appear in myriad colors and leave that slimy goo on your pool walls and floor.
Algae collects most rapidly in plaster pools because its rough surface gives the algae something to cling to while it's growing, but it also appears in all types of pools, sometimes regardless of the good care and cleaning you perform.
Being proactive with algae is the best way to prevent its growth in the first place. You need to be diligent about sanitation and cleaning. Be advised, though, that even if you are fastidious about the cleaning, and even if you keep the pool water clean and the chemicals in balance algae still happens! Algae causing spores could be carried in from the water use used to fill the pool and can even be carried in on the wind.
You'll find that algae is most prevalent in shallow water because the water there heats up faster than deeper water and the warmth is what the algae needs to thrive. Pool steps are a favorite breeding ground for spores so pay special attention to them when you're performing your maintenance.
Algae like most molds can be hard to remove once it's taken hold. While some can be easily brushed or scrubbed away, others take more elbow grease and concentrated effort.
If you find algae taking hold despite your best efforts you'll want to check with your pool chemical supplier for an algaecide that can help rid your pool of the green growth.