Gravity is the force of attraction that attracts a body towards the centre of the earth or towards any other physical body having mass. You will get enough information about this universal phenomenon when you type in 'Newton's law of Gravity' in a search engine. Friction on the other hand is the resistance of motion that an object or a surface encounters when moving over one another. It is a part of Newton's second law of motion. One should know that gravity plays an important role in gravity. Speed is nothing but distance travelled per unit of time. One should also note that speed is just a scalar quantity which means that the speed of a body will tell you how fast a particular object is moving. It will however, not tell you the direction in which an object is moving. To find out more about these three concepts i.e. Gravity, friction and speed, you can log on to www.physics.org.