difference between UART ans USART communication. can we send video from using this method of communication?

A simple google search resulted in this

UART ( Universal Asyncronous Receiver Transmitter ) USART (Universal Syncronous Asyncronous Receiver Transmitter )

The USART can also work in a Syncrounous mode , you send a clock along with the data. They can usually work quite a bit faster than the 115Kbd of normal Uarts. You normally see USARTS built into Micro Processors more than stand alone , but there are a couple of stand alone devices around.

can we transmit video signal from cmos camera to computer using UART

yes if you have a camera that works with UART

Yes, you can send digital video data over a UART or USART, but the bandwidth is so low that you won't be able to get very high resolution. And you'll need to connect a special video camera that will send serial data. Some very old web-cams did this by putting video compression hardware inside the camera.

UART = Universal Asynchronous Receiver/Transmitter; USART = Universal Syncronous/Asynchronous Receiver/Transmitter. So, a USART adds the capability to do synchronous communications.