Explain the video standards PAL, NTSC, and SECAM, what are the main differences between them

PAL: Stands for Phase Alternating Line, is the analogue video format.The name "Phase Alternating Line" describes the way that part of the colour information on the video signal is reversed in phase with each line, which automatically corrects phase errors in the transmission of the signal. 

NTSC: The National Television Standards Committee. The NTSC format consists of 29.97 interlaced frames of video a second, each consisting of 480 lines of vertical resolution out of a total of 525 (the rest are used for sync, vertical retrace, and other data such as captioning)

SECAM: (Sequentiel Couleur avec Mémoire, French for "sequential color with memory") is an analog television system, using frequency modulation to encode chrominance information. It is so named because it uses memory to store lines of colour information, in order to eliminate the colour artifacts found on systems using the NTSC standard.