During the normal course of play, you aren't likely to type many slash commands that are generally useful for macros. Sure, the occasional emote macro can make for some interesting role playing, but c'mon... There's got to be more to it than that...

There is. Enter /cast, the most common command you will see in macros. The /cast command allows you to cast any spell from your (or your pet's) spell book by name. The simplest case is a command like:

/cast Shadow Word: Pain

This macro will cast your highest-rank Shadow Word: Pain on your target. It behaves exactly as if you had dragged SW:P onto that spot on your action bar. The action bar code recognizes the spell and will show cooldown and range feedback on the icon. In fact, if you choose the question mark icon I mentioned earlier, the action bar will even show the icon for SW:P.

"Ho, hum," you might be thinking... Why not just put the spell on your bar? Well, that's where combining multiple commands comes in handy, and this is exactly what makes macros so useful. What if you're a mage and you want to let your party know that you're about to sheep something? Well, simply put the cast & /p message in a macro (there are better macros for this task--[insert shameless plug for my CCWarn (http://www.wowinterface.com/downloads/info6826-CCWarn.html) addon here]--but this is a nice, easy to understand example):

/cast Polymorph  
/p Sheeping %t! You break it, you tank it!

Note: Since the macro is executed all at once, the /p command will be issued when you start the cast, and will not care either way whether you have a valid target or whether Polymorph is on cooldown. This also means you can put the two commands in either order and it will have the same effect. If you want to say something only when you actually cast the spell, check out the addon AfterCast (http://www.wowinterface.com/downloads/info4167-AfterCast.html). AfterCast schedules a slash command to run... well... after you cast a spell (within the limitations mentioned at the end of "What is a macro?"). For example:

/aftercast /p Click the portal to get %t's lazy butt over here  
/cast Ritual of Summoning