ONVIF defines a standardized interface to send commands to an IP camera. It also endorses a behaviour of the camera through its tester, so yes, you can expect that if you interoperate with a camera then you'll probabibly interoperate also with other ones.
The ONVIF specification can be found here.
Finally, let's say that HTTP programming is in general not enough. You'll have to deal with a SOAP-defined protocol, and coding it by hand is, imho, crazy: never write a parser by yourself! (unless you really know what you're doing). Thus you'll need to make use of gsoap or visual studio or any other framework that generates encoding and decoding for you.