Hybrid golf clubs are here to stay.  If you are thinking about buying one or two but don't know what to look for then let me help you.  First of all, hybrids (sometimes referred to as utility clubs) have been around for years but only in the last few gained popularity among the professionals.  The reason has as much to do with the golf balls as the clubs!  Modern golf balls spin at a lower rate.  This is great to increase distance off the tee (High launch + low spin = distance), but very bad if you need to get the ball in the air with a low lofted iron.  Spin creates lift and without it even the best players will find it almost impossible to hit a long iron off the turf.

So, now we have hybrids.  Or as some call them "cheaters" because they are so much easier to hit!  Hybrids have a longer shaft than an iron but not quite as long as a wood.  The head is bigger than a long iron with a wider sole.  It is the head size, shape and weighting that helps get the ball in the air more than the loft.  What I mean is that if you have a 22 deg lofted hybrid and a 22 deg 3 iron, the hybrid will have a higher trajectory.  Without getting too technical, because of the bigger head, the center of gravity is farther away from the clubface.  This equals greater effective loft at impact.

Ok, which hybrid should you get and how many should you have?  Here are a few things to consider:
#1  Know yourself.  If you don't know what your swingspeed is then you need to find out.  We can estimate by what club you hit 150 yards.  Be honest.  If it takes a 5 iron or more, your swingspeed if 60 mph or less.  If it is a 6-7 iron, you are about 65-75 mph.  If it is 7-8 iron, you are 75-85 mph and 8 iron or below, 85+.  
5 iron or more - You shouldn't be hiting more than a 5 iron if that!  Start with a 4 iron replacement (27-25 degs) and go from there.  You may find that actual fairway woods work better than hybrids.  The slower your swingspeed, the more help you need from the club. 
6-7 iron - You are the target audience for hybrids.  A 4 iron replacement (27-25 deg) and a 3 iron replacement (24-21 deg) would almost certainly help your game.
7-8 iron - You might have the speed to hit a 4 iron but a 3 and 2 iron replacement (20-18 deg) would probably be a good idea.
8 iron or less - You can probably still hit your long irons but why?  Even Tiger Woods says that the days of the 1 iron are gone and the 2 is not far behind.

# 2  What type of head shape should you look for?
Hybrids are just that, a hybrid of an iron and a wood.  Some are more one than the other.  The best way to know for sure is to demo a few at the driving range or golf store.  If you can't do that (or even if you can) keep this in mind.  If you hit irons better than fairway woods, look for a smaller head with some offset.  Offset is the face of the club set behind the shaft, common in almost all irons.  You can hit these clubs just like an iron but with better results.  If you prefer woods over irons, look for a bigger head shape and little to no offset.  You would play these like a fairway wood.
One quick thought on the shaft.  Obviously it should be a flex that matches your swingspeed (that's a whole other topic!) but the material makes a difference too.  Most people with a lower swing speed will want a graphite shaft.  It will be lighter, longer and have a smaller tip diameter.  All of which help to get the ball up and flying.  A steel shaft works well for those with higher swingspeeds who want a little more control over the ball.

So ditch those hard to hit long irons and get yourself a hybrid or two.  If you don't like them, you can always sell them (or resell them) on eBay.  Thanks and keep it in the short grass.