I originally wasn't planning on devoting an entire page on leash and collar selection... then I saw someone in my neighborhood trying to walk her Great Dane and her German Shepherd at the same time, a leash in each hand.  They both excitedly tugged in different directions to the point where I'd be surprised if she stayed on her feet through the entire walk.  If she had a "Take Two Adjustable Leash" then the dogs would have mainly been pulling eachother, and even better, only one of her arms. 

When you're picking out a leash, make sure you get the leash that's the best for your situation.  For instance, if you have 2 dogs and only one person to walk them, consider looking into the leash I mentioned above.  If you live on a busy street, stray away from super long leashes and retractable leashes.  You'll find them in the street before you have time to react.  The ideal length for the leash is about 6 feet.  This length will give you enough control while at the same time allowing them a peice of freedom to roam about.
Petmate by Aspen Pet Take Two 5/8

Flexi Classic 1 Cord Retractable Leash in Red (16' Length; Red; Cord)

Pet Tags Lost Pet Recovery System - Brass Bone (1 1/2 PETCO 3/8 Petmate Signature by Aspen Pet 5/8 Petmate by Aspen Pet Mighty Link Light Weight Chain Collar (2mm Diameter; 12 AKC Large Navy Blue Padded Dog Halter (Large; 1 So long as I'm talking about leashes, might as well talk about collars, as well.  Collar selection, while not as important as leash selection, is still a real item for consideration.  

First, I need to say to always make sure you can fit two fingers under your dog's collar!!  A tight collar can and WILL cause a ton of discomfort.  The tight collar will quickly rub the fur off of your dog's neck leaving behind a painful, sometimes even bloody, rash. 

A good way to measure for collar size is by using a string or shoelace or even a telephone or networking cable to measure, then laying it out strait to a ruler. 
   
The traditional, ring around the neck collar is the best "in general" collar.  They will always have a spot for a name tag which you should always fulfill.  The traditional collar is very comfortable for your dog, while at the same time giving you a lot of control.  It is recommended that every dog you own have a traditional collar for general wearing purposes.

There are other types for special purposes, however.  If your dog has problems with it's throat, a harness is recommended for walks. 

A Chain-Slip collar can be used for training purposes. Again, if your dog has throat problems of any sort, this type of collar is NOT recommended. Giving a short, light jerk on the collar can be very uncomfortable for the designated dog, yet safe. This is a good method to help get rid of undesired behaviors. 

Another option is a halter. If your dog is too large for you to control, and/or is aggressive towards people or other animals, this might be the leash for you. It basically acts as a muzzle. The leash connects to the halter under the neck, forcing any resistance you give the dog to pull their head down or to the side, giving you complete control. 

Another option for training, is the pronged collar.  I would consider this a very last resort method.  It's unnecessarily uncomfortable for serving the same basic purpose as a chain-slip.