Accuracy & ToHit
They are two variables in the maths used to judge if you hit something you use a power against.
For complex mechanics like this, you head to the Godfather of CoH info...
http://paragonwiki.com/wiki/Attack_Mechanics
Essentially for all the layman needs to know (like myself), ToHit bonuses are more powerful (unit for unit) than Accuracy bonuses.
Thanks.

Also, it's important to realize that your final to-hit is basically (in simplified form) equal to your base to-hit (depends on mob level, it's 75% for +0 and 48% for +3 for example) plus any to-hit bonuses, multiplied by your accuracy. (Assuming you aren't debuffed)
That means that accuracy effectively enhances to-hit bonuses since you add the to-hit up before multiplying by accuracy. If you want to hit really hard targets like +4s or high defense foes the best thing you can do is get to-hit buffs and significant accuracy bonuses.
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
The Scavenger Hunt: 187076
The Instant Lair Delivery Service: 206636
Also: every single debuff you encounter in the game is a Tohit debuff, regardless of what the text is telling you. The game doesn't have Accuracy debuffs anywhere. (At least, not yet.)
http://www.fimfiction.net/story/36641/My-Little-Exalt
Until recently, I was under the impression they were the same thing. I was curious if some of the "experts" could explain what the difference is between the two.