Algae (known as phytoplankton by scientists) are microscopic plants that can grow in almost any type of conditions but warm swimming pool water is a particular favourite for them. Algae spores can blow in to your swimming pool from almost anywhere and if they find the water to their liking you can very soon have an algal bloom in your pool.
The conditions that algae like are warmth and light and food to eat. Cut out any of these and you will control or eliminate algae from your pool. If you can't cut these out then that is where algaecides can help you out.
Algae control in the summer
In the summer warmth is pretty much a given not just from you heating your pool but from the ambient conditions also. Light is a problem too because the days are 60% longer in summer compared with winter in the UK. We like the sun to shine on our pool water surface to help warm it up. So that just leaves food. Algae eat phosphates. Get rid of phosphates from your pool water and algae cannot survive. The chemical called Starver does just this. It reacts with phosphates and changes them to something else that can be taken out by your filter. Removing phosphates is a fairly new technology, the most popular way control algae in a swimming pool is to use an algaecidal chemical. Most of these are based on copper sulphate. Copper kills algae. Strictly speaking it makes algae infertile so it can't reproduce and therefore dies before replicating.
In the winter two of the main factors are less prevalent. Warmth and light. It does not matter (for an outdoor pool) if the water is cold. Putting a cover on the pool that keeps the light out will make a big difference. In fact blocking light from your pool in winter is all you need to do keep your pool clean until spring time. If you don't have a cover that keeps light out then you are back to relying on chemicals. Winter algaecides work in much the same way as summer algaecides by adding copper to the pool water but they last a bit longer before needing topping up.
Removing algae from a swimming pool
If you already have a green algae bloom in your swimming pool then you are not really in the right section. To kill and remove algae that has taken hold you need chlorine. In particular you need shock chlorine to kill algae. Chlorine is more normally used in small doses to target bacteria but at increased strength it is the only way to kill algae. Once you have dosed your pool with chlorine the algae will die and turn from green to white. This makes your pool water look cloudy so you will need to filter out the dead bodies to get the water looking clear again. Once you have removed all traces of algae in the pool you can start back using algaecides to keep the algae from returning.