H264

From Wikipedia:()

(H.264/MPEG-4 Part 10 or AVC (Advanced Video Coding) is a standard for video compression, and is currently one of the most commonly used formats for the recording, compression, and distribution of high definition video.

 H.264 is perhaps best known as being one of the codec standards for Blu-ray Discs; all Blu-ray Disc players must be able to decode H.264. It is also widely used by streaming internet sources, such as videos from Vimeo, YouTube, and the iTunes Store, web software such as the Adobe Flash Player and Microsoft Silverlight, broadcast services for DVB-S2 and SBTVD, HDTV direct-broadcast satellite television services, HDTV cable television services, and real-time videoconferencing. Digital Video Broadcasting for Satellite DVB-S, Cable DVB-C, and terrestrial broadcasting DVB-T use MPEG-2.)

True compression and IFrame

The main difference with video compression vs. still image compression is the reduction of Temporal Redundancies. Temporal Redundancies are the similarities between images. A significant amount of storage and bandwidth, or bitrate, can be saved by not retransmitting the macro blocks which don’t change from frame to frame.
        In a security application this can refer to the background of the image. Many cameras in a video security deployment are fixed cameras. The only thing changing in the camera’s field of view are vehicles passing, people walking etc. The majority of the scene from image to image is the same information (the background). True video compression techniques, like MPEG4 part 2 & H.264, take advantage of this fact and only transmit the background periodically. The result is a video stream composed of a reference frame, called an I Frame, and then changing areas of the image are transmitted and overlaid on the original reference frame to create the current image of the scene The frames with changes are referred to as P or B frames.
        An I Frame followed by a series of P and B frames is referred to as a Group of Pictures or GOP The number of frames from one I frame to the next I frame in the stream is referred to as the GOV Length.