You should probably figure out what you need a macro to do for you first...
Here's a start, read my old Blog post on Macroneering - When to Use Macros.
Here's a great macro. Not very useful though.
#showtooltip Lifegiving Seed
/use Lifegiving Seed
/s By the power of Greyskull, I am HERBMAN!
Mouseover macros are simply macros which involve using what your mouse is currently hovering over in it's targeting rules.
That for example will attempt to cast Heal on whoever you are mousing over at the time, provided they are friendly and not already dead. If it can't use that target, it will try and use your current target's target (i.e. if you are targeting a boss, you heal who the boss is targeting). If that doesn't work, it will try using your current target. If that fails, it will just cast the heal on yourself. This is a relatively complex mouseover macro cause it includes a lot of other cases in it, however is still a mouseover macro because it uses that as its highest priority.
/cast [@mouseover,help,nodead] [@targettarget,help,nodead] [help,nodead] [@player] Heal