What can you do with UART ports in Microcontroller?
UART is a communication protocol. If micro controller wants to communicate to a specific sensor which supports UART then the micro controller can be interfaced to that sensor using UART protocol.

In your micro controller you have specific Tx(Transmitting) and Rx(Receiving) pins. If you go through your controllers datasheet or pinouts you will get it.
OV7670 works with UART interface. Your connection should be like this
OV7670 Tx----Micro controller Rx
OV7670 Rx----Micro controller Tx
OV7670 Gnd---Micro controller Gnd  

If you are going to connect the micro controller to PC or any other RS232 devices then you may need to convert TTL logic to RS232 logic. You have many readily available converters for this purposes.