There are two different types of nebulizer designs.  The most common is the compressor nebulizer.   This type uses an aerosol compressor to vaporize droplets of medicine which are inhaled through the use of a nebulizer kit and mouthpiece (or sometimes a mask).  

The other design type is the Ultrasonic nebulizer (or Mesh nebulizer).  This design uses high-frequency sound waves to turn liquid medicine into a mist which is then inhaled using the same nebulizer kit.  The results are the same, but Ultrasonic nebulizers tend to be quieter, more portable, and usually deliver medications quicker for shorter treatment times.  

As you would guess, there are many different nebulizers on the market and it is a little confusing and sometimes difficult to choose which is the best for you.  We would suggest that the first thing you decide is where and how often you are going to use your nebulizer.  

If you will always be taking your treatments at home, then you can use either type of machine. You'll find that compressor nebulizers are the least expensive for this reason.  

If you need to use your nebulizer in the car or on a camping trip, then you will need a battery powered option.  Most Ultrasonic units have a battery option available, but can still be used while plugged into the wall.  They are smaller, quieter, and easily fit into a purse or small bag.  As you would expect, they are also more expensive than compressor nebs for that reason.