For questions like this, go to www.google.com and type in words like: natural convection and forced convection.
I just took those very words from your question, put them in google, and in the first results, found the answer:
The temperature of the solid due to an external field such as fluid buoyancy can induce a fluid motion. This is known as "natural convection" and it is a strong function of the temperature difference between the solid and the fluid. Blowing air over the solid by using external devices such as fans and pumps can also generate a fluid motion. This is known as "forced convection".
What this means is: Say you have a bowl of hot soup. If you leave it alone, it will cool off by natural convection because the air around it is cooler than the soup. If you blow on it to cool it off, that is forced convection.
To find pictures, you also go to www.google.com and put in the same words, but click on the word "Images". Then pictures will come for the thing you want. It's just amazing. Good luck. Use google all the time.