Additionally, you can make macros that utilize the <tt> command. Example:

/micon "cure"
/ac "cure" <t>
/ac "cure" <tt>

What this does is if <t> isn't valid, then <tt> will target your target's target. Handy when you want to dps without taking your focus off your tank, or if a dps is getting chased, you can target the mob and heal its target, etc.