How add Hotel booking Services to existing Hotel reservation software

hi

i have writen hotel reservation software. and is's installed on some hotels.

i know there are some hotel booking site like :

 http://www.hotelscombined.com/ 

http://tr.hotels.com/

https://agoda.com/

now i want to know how this sites work? how they talk to the hotels around the world?

how can my software connect to these site and the visitors of these sites can reserve my customers hotel?

any suggestion

thanks.