Phase detect AF points are actually lines which detect horizontal, vertical or diagonal lines. The normal AF points are generally vertical lines which detects horizontal lines on your subject to determine focus, while the cross-type combines vertical and horizontal AF lines which will detect both horizontal and vertical lines on your subject. The dual-cross points have an additional X shaped lines across the normal horizontal and vertical AF lines, which can help get even better precision.
http://photographylife.com/how-phase-detection-autofocus-works seems to be a decent summary of how Phase detect AF works.
Now Phase detect traditionally cannot be used for AF during Live-View or Video as it requires the mirror to be down to bounce a small bit of light down into the bottom of the mirror box which is where the AF sensor sits. The way it does the Quick AF during Live View is it drops the mirror down briefly, does it's auto-focusing, then pops the mirror back up. Otherwise, during Live View it uses Contrast Detect which is a more accurate, but slower way to get the correct focus. However, you currently can't use that during video recording on Canon DSLRs, excepting the T4i. An advantage of Phase detect over Contrast is that Phase detect can tell you the direction you need to move the focus towards (near or far), while Contrast has to hunt in one direction, and if it doesn't get focus hunts all the way in the opposite direction.
Onto the T4i, that camera happens to have some phase-detect AF sensors put onto it's main sensor, which allows it to use Phase detect to help determine the direction it needs to move the focus in, and Contrast detect to fine tune it and achieve focus. This is likely similar to how the EOS-M does it, since it has the same sensor in it as the T4i.
Now, if we are talking only stills, the Phase detect which is always used when you look through the viewfinder (the mirror is down) or when using Quick Focus in Live-View is generally what is used. It tends to be faster, and usually reasonably accurate. However, if you want extremely accurate, you need to switch to Live View and use the Constrast detect AF. However, this is usually best done from a tripod, unless you have a very large Depth of Focus/Field because hand-held, especially holding it out in front of you to view the Live View, you generally are less steady and have more hand movement. This means you're more likely to move your subject out of focus, especially if it is near to the lens and/or you have a very wide aperture (small f-number).
