SECAM is like PAL in that a 64uSec delay-line is used to store a previous line of chroma (colour) information.
In SECAM, that remembered line is used to fill in the next line of colour information, thus halving the amount of chroma information that has to be transmitted and reducing transmitted bandwidth. There are more lines per picture in SECAM than PAL or NTSC, so they had to skimp on chroma bandwidth to preserve bandwidth for the Luminance (black and white) component of the picture.
The difference between PAL and SECAM lies in how that stored chroma information is used.
In PAL, the information stored in the delay-line is used to compare to the next line, and is able to substantially cancel any gross phase-errors in chroma information that inevitably arise from the vestigial sideband transmission techniques used for terrestrial TV brroadcasting.
In this manner, PAL TV receivers give very accurate rendition of colours without the need for a "Hue" control that used to be fitted to NTSC receivers.
The PAL Chroma signal is actually two components transmitted in one composite signal. Black and white information is called Luminance (abbreviated to Y). There are two colour-difference signals:- Luminace minus red, and Luminance minus blue. There is a third (Luminance minus Green), but if you know Luminance and you know the other two, this third colour difference signal can be mathematically regenerated in the receiver, which is exactly what happens (in a circuit called the PAL Matrix). In transmission, Luminance and two colour-difference signals are transmitted, all squeezed into about 7.5Mhz of bandwidth (in the Aussie PAL system at least).
The amplitude of the chroma signals determines the saturation of the colour, the phase (or timing with respect to a reference colour signal called the colour burst) determines which colour is displayed.
The purpose of the internally-generated colour-burst is to re-synchronise the chroma decoder at the start of each new scan-line, assuring accuracy in colour rendition. It is complex, but it works magnificently.
NTSC was the first colour system, and when it was designed, the engineers knew the principle of a memory holding the colour information from a previous line to compare it to the lext line and cancel phase-errors, but the actual physical device (the 64usec delay-line) didn't exist, making them unable to use the principle in their system.
So they cheated - a bit. They also used two colour-difference signals, but they went about it in a very clever way. Since the colour response of the human eye is very poor in the purple part of the spectrum, they deliberately "tilted" the vectors of the two colour difference signals (which they called I or "in-phase" and Q, the Phase-Quadrature signal which is 90-degrees out of phase with the "I" signal) about 33 degres into the purple part of the spectrum. By doing so, small phase-errors in chroma were hard to notice. When the phase errors were gross, the colours looked crook, to use a good old Aussie term.
NTSC receivers today seem to have done away with the "Hue" control, but if you were watching NTSC and the faces appeared grey, you went and tweaked the "Hue" control until the faces were a more pleasing pink, or whatever you wanted.
Of course, with digital terrestrial broadcasts, all this is now old-hat, but in its day it was a great technological achievement.
In TV broadcasting PAL jokingly stands for "Perfection At Last", NTSC stands for "Never Twice the Same Colour" and SECAM stands for "System Essentially Contrary to American Methodology". (This may explain why the Russians adopted it!) TYhere is a small grain of truth in these jokes actually.
In Argentina they use PAL-M...unique to that country. An Argentinian engineer I spoke to said it stood for "Pay A Little More" because all their sets seemed to cost more.
Hope this helps.