if you clone a github repository using the https:// method (instead of ssh), you will have to type your username and passwor all the time.
In order to avoid having to type you password all the time, you can use the credential helpers since git 1.7.9 and later.
git config --global credential.helper "cache --timeout=7200"
means “keep the credentials cached for 2 hours. (default is 15 minutes).
You can also store the credentials permanently using
git config credential.helper store