A Few Things About Fishing Reels.
This page features a few things to be aware of and look out for when buying fishing reels. You needn't spend a fortune, I've seen some very good reels on the market for under £20. This site isn't sponsored by them, but Okuma make some excellent, well priced reels. I've also used Shakespeare and Daiwa reels in the past and had good value for money. There are also many other good reels on the market.
When choosing a reel make sure that the handle turns smoothly, also that the anti-reverse is positive. When you switch the anti-reverse on, the reel handle should not have any backward movement. Also test the clutch, try it at different settings to make sure it works correctly. If it doesn't, it could cost you lost fish.
Fill The Reel Spool Correctly.

If you only take away one thing from this website let it be this. Always fill your reel spool fully with line. It wants to be full like the example on the right. Unless you do this you will not be able to cast correctly and fish effectively. It's amazing the number of people I see on the bank, with a reel with a big deep spool. They hardly have any line on it and yet they are trying to cast a small float out - it just doesn't work.
On the other hand make sure you don't overfill the reel, or else line will be constantly falling of the spool during casting etc. I always like to fill the reel to within an eighth of an inch from the lip of the reel spool. Any more will cause problems.

Choosing A Reel With The Right Spools.
If your choosing a reel for general fishing, that is float and ledger, don't choose a reel with a deep spool. I've mentioned above about the importance of filling your reel spool correctly.
The two spools on the right came with a reel that I am using at the moment. The spools are designed to be as shallow as they are for a very very good reason. Tackle shops sell line in 100 meter spools. The reel on the left is for lines of about 2 to 3 pound breaking strain, a 100 meter spool of such line will just fill it up correctly. This is the spool I would more that likely use for float fishing. The spool on the right is just a little deeper. As I'm sure your aware as the breaking strain of line goes up so does it's thickness or diameter. So the spool on the right will just take 100 meters of line about 4 to 5 pounds breaking strain. This is the spool I would probably use for ledgering. Naturally carp reels have to be deeper, as you are dealing with lines 8 pound breaking strain plus.
Pay great care to the diameter or depth of the spools when choosing a reel. The really good ones for general coarse fishing will give you spare spools, with different depths as above.
Float and Ledger Reels.

The above picture shows a typical coarse fishing fixed spool reel that can be used for float or ledger fishing. It is made for use with lines of breaking strains between 2 to 6 pounds. It's called a fixed spool reel, because the reel drum itself does not rotate, instead the bail arm travels around the spool, feeding line onto it. I will go through some of it's features:
1 - Clutch.
Having said that the fixed spool doesn't rotate, it can be set to do so by the clutch when playing a fish. The clutch sets the force of pull on the line that's needed for the fixed spool to rotate and give line, when fighting a fish. So the weaker your line, the slacker the clutch needs to be set, so a fish doesn't break your line, if it makes a sudden run away from you.
Some reels have the clutch adjuster at the rear of the reel and set by a dial, like the one above. Others have a clutch at the front of the spool. Which you choose is a matter of personal preference, there are advantages and disadvantages in both. What really matters is that the clutch should be smooth in operation, so it can give line evenly when needed.
2 - Anti-Reverse Lever.
Nothing complicated here, it stops the bail arm from travelling backwards to take line of the spool. It is normally found underneath the reel, like the one shown above. The thing to look out for when buying a reel is to engage the anti-reverse and then make sure you can't turn the handle backwards by anything other than a fraction of a turn. It's one of the signs of good engineering in a reel, in others words it's probably well made.
3 - Bail Arm.
The bail arm has 2 positions, open and closed. The bail arm in the above picture is open, which allows the line to come freely of the spool during casting. It is closed by turning the reel handle, which activates a spring mechanism to snap it shut. When buying a reel test the bail arm to see it closes properly with a good firm "snap".
4 - Push Button Spool Change.
One of the advantages of having the clutch control at the back of the reel, is that the spool has a button at the front, that you push to release it. It makes changing spools much easier. Again something to test when you are looking at a reel. Maker sure that when you press the button the spool pulls away easily. Engage and re-engage the spool a few times just to check the mechanism.
Carp Reels.
I think what I have said above about what to look out for applies to carp reels as well. The spool in this case needs to be deep to accommodate lines of 8lb or over breaking strains. Make sure you get spare spools, so you can have different strength line loaded on each, to cater for different fishing conditions.
Baitrunner Reels.
If your going to buy a carp reel make sure it has a baitrunner mechanism. Different manufacturers call it different names such as "bite and run", "free spool" etc.

Carp will often pick a bait up and bolt of at great speed, it is know as "a run". The speed of the runs are such that a rod could easily get pulled in, even by a very small carp. Believe me, I've seen it happen many times, even to some experienced anglers.
The idea behind the baitrunner is to allow you to carp fish with the bail arm closed, as in the picture above. They all have a baitrunner lever, which when you set to free spool, allows the spool of the reel to turn freely. This means that when a carp bolts off at great speed, it can take line freely of the reel. To strike and play the fish you turn the reel handle, this disengages the free spool and the reel now acts as a normal reel.
The reel should also have a bait runner adjustment mechanism, which is normally at the rear of the reel. This allows you to set the tension of the free spool. For example if you were fishing a some distance and needed a heavy bobbin on, if the baitrunner were set to loose, it would pull the bobbin down to the ground all the time. The baitrunner adjustment allows you to just give the free spool enough tension, so the bobbin is help up between the rod and ground.
When buying a baitrunner reel always check that the baitrunner mechanism is working correctly a few times. I had one fail on me in a shop when testing a reel. Needless to say, I picked up another reel and gave that a good test, which proved OK.