Weak Auras is definitely not the only addon that can do this but it is an extremely useful addon, in my guild it is almost compulsory.
It can track and alert you of just about every CD, Buffs & Debuff's, Proc's, Boss Abilities and specific timers. You can set it up yourself to look exactly how you want and to fit your UI, personally i just find strings that i think would suit me, upload them and edit them slightly if needed.
http://howtopriest.com/viewtopic.php?f=13&t=2722
The above link is a decent place to start for some Weak Aura strings you may find useful for your priest but there are tons out there so do some searching till you find ones you like.