What is difference between 1080 and 720?

Both 1080 line and 720 line picture formats are HD signals.  720p is a lower spatial resolution but a higher temporal resolution than 1080i broadcast signal.  That means that the 1080 line image is refreshed at half the rate of the 720p signal.

720p is regarded as a better format for high speed movement, hence it has some popularity for some sports broadcasts although 1080i is by far the more common format for broadcast television.

Note that 1080p is not a format that is being broadcast at present and for the time being it is limited to local sources such as Bluray players and games consoles.