Portability
Portable washers are small so they are easy to store, and can be rolled around on casters when you're ready to use it. They also come with a faucet adaptor so they can be hooked up to any standard kitchen faucet for water.

Wash Method
There are two main methods of washing in portable washers: agitator and pulsating. Washers with an agitator are much like the traditional top-loading washer you're used to, that rotates the clothes back and forth to clean them. Pulsator washing has no agitator but instead "pulses" water through the clothes to clean them more gently. This is better for your clothes and will make them last longer, particularly delicate materials.

Single Tub vs. Twin Tub
Twin tub washers have a separate basket just to spin dry the clothes. This enables the machine to reach much higher spin speeds and dry more effectively than traditional single tub machines.

Wash Cycles and Controls
Some machines offer a handful of different cycles to choose from, and some only offer one or two. Additionally, some machines have manual controls (for instance a knob) and some have digital controls (soft touch buttons) and even LED display! The digital control units will often also have a timer, so you set your clothes to wash while you're out and then be home in time to get them out.

Water Usage
Some machines use much more water relative to their capacity than others. Look for a machine that uses less water to save money and help the environment.