What Fu.Th.Sy. (is that a contraction of what I think it is?) said.
Rally restores part of the morale of friendly troops, including those who have already fled the battlefield. Quite amusing to see an enemy general with the Confident trait and very poor Martial skill; they keep rallying their troops, only for more of them to be slaughtered.
Wedge, Envelope and Force Attack all improve your army's killing ability. Not 100% sure of which value they improve (Offensive or Discipline?), but they make some or all of your troops more stabby. Wedge improves central units, Envelope improves flanking units. Hold Ground does the same but for Defensive, IIRC.
Rally adds morale to friendly units, including those who have already retreated off the battlefield. Potentially a very powerful ability, but it can also hurt: I've seen AI generals with poor Martial skill repeatedly rally their troops only to get more of them slaughtered by my army. So, be careful about giving poor generals with the Confident trait command of an important battle.