I am not a philosopher, but I will try to simplify it.
Existentialism is the belief that life only has meaning in relation to the individual person – his emotions and thoughts. Also, it is the individual who is responsible for giving his life meaning in spite of all the opposing forces that get in the way – boredom, alienation, etc. Most existentialists did not believe in a higher power but those that did believed God may have created man, but then he left man to his own devices. There are lots of varying degrees to this belief, so this is a simplification.
Absurdism is a more extreme philosophy. Absurdism is the belief that there is no meaning to life, or no meaning in the universe, so that man’s attempts to search for such meaning are “absurd” because that meaning does not exist. Some absurdists believe in a slightly different view, that there IS meaning, but that man cannot find it, and the only thing that gives meaning to life is absurdism itself and that man should embrace the fact that life is absurd if he wants to survive. This was the view of Camus. He believed man should embrace the absurd and live in spite of it. If man could do this, this was all he could hope for in life.
Pretty grim stuff. This philosophy was in many ways an outgrowth of the pessimism inflicted upon the world after two horrific world wars that left footprints of hopelessness on the world.
If you do some research, you can define these terms even further.