Mouseover macros will cast your spell on what ever target your curser is over. This works for unit-frames and for putting your cursor over the actual character/mob.

Mouseover is really nice for healing, not such a big deal for DPS. Healers are often switching targets quickly, heal person A, then person B as soon as the first heal is finished. I raid as a resto shaman and I can tell you that using mouse-over is a great help when healing with the raid frames addon Grid or similar mods. The main reasons I like it is I feel that I can switch targets for healing more quickly, instead of having to click to target the person and then heal, I just mouseover them on Grid and cast my spell. Also I was having wrist pains after moving my mouse around and clicking constantly for 3-4 hours, 4 nights a week.

Here is the exact healing macro I use on my shaman.
[code:1]#showtooltip Chain Heal(Rank 5)
/script UIErrorsFrame:Hide()
/use Essence of the Martyr
/cast [target=mouseover, help] [help] [target=targettarget, help] [target=player] Chain Heal(Rank 5)[/code:1]
The macro does the following:

Uses a trinket, does NOT show the 'item is not ready' message

Casts a heal on whomever I am hovering my cursor over.

If no mouseover it will - Heal target of target if my target is hostile, enables you to target the boss and always heal whomever has aggro.

If no mouseover AND no unfriendly targeted - Heal friendly target.