Analogue video has been the industry standard for many years and is generally supplied as PAL, NTSC or SECAM analogue encoding.
Each frame of an image is composed of lines drawn on the screen. The lines are of varying brightness; the whole set of lines is drawn quickly enough that the human eye perceives it as one image. The next sequential frame is displayed, allowing the depiction of motion. The analogue television signal contains timing and synchronization information so that the receiver can reconstruct a two-dimensional moving image from a one-dimensional time-varying signal.
Analogue broadcast systems come in a variety of frame rates and resolutions. However there are three standards for the way the additional colour information can be encoded and transmitted. The first was the American NTSC (National Television Systems Committee) colour television system. The European/Australian PAL (Phase Alternation Line rate) and the French-Former Soviet Union SECAM (Séquentiel Couleur Avec Mémoire) standard were developed later and attempt to cure certain defects of the NTSC system. PAL's colour encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC.
In principle all three colour encoding systems can be combined with any scan line/frame rate combination. Therefore, in order to describe a given signal completely, it's necessary to quote the colour system and the broadcast standard as capital letter. For example the United States uses NTSC-M, the UK uses PAL-I, France uses SECAM-L, much of Western Europe and Australia uses PAL-B/G, most of Eastern Europe uses PAL-D/K or SECAM-D/K and so on.
However not all of these possible combinations actually exist. NTSC is currently only used with system M, even though there were experiments with NTSC-A (405 line) and NTSC-I (625 line) in the UK. PAL is used with a variety of 625-line standards (B,G,D,K,I,N) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.
For this reason many people refer to any 625/25 type signal as "PAL" and to any 525/30 signal as "NTSC", even when referring to digital signals.