Some applications aren't ended completely when you return to the home screen. If you don't end them from the list of running applications, they keep running in the background which may slow down your phone.
Step 1 of 4
Press the Application key.
Step 2 of 4
Slide your finger right starting from the left side of the screen.
Step 3 of 4
To end one running application, slide your finger upwards on the required application.