Like what Jasmine has mentioned, throwing things is part of the learning process for a child. He/she is not doing it to irritate you but to more for him/her to learn about the different textures (and the cause and effect) of a certain object. It is very common for babies to throw food on the ground or on the table as this will allow them to experiment and learn more about the food as it lands onto the food or table. Yes, it can be messy and frustrating for the adults (especially since we are the ones doing the cleaning.. sigh). Just bear in mind that it is for their learning to make yourself feel better. You can also enhance his/her learning by exploring how the object (hopefully not food) lands, whether it rolls or bounce or goes splat. Makes for a good educational play session as well. :)
Throwing things is a new and enjoyable skill for many children between 18 months and 3 years of age. It takes fine-motor skills to open the fingers and let go of an object, and considerable hand-eye coordination to actually throw it. Your LO just wants to practice this exciting skill! This skill is also educational for your LO, as he/she discovers that whatever he/she throws it falls down — never up. He/she its observing its effect of gravity. If he/she throws a ball, it bounces. If she tosses a plum, it goes splat. It's nothing worrying about and it's futile to try to stop your child from throwing at this age. Concentrate instead on limiting what he/she throws and where he/she throws it. http://www.babycenter.com/0_throwing_1200989.bc