Online galleries work much like their real life counterparts. These online galleries display an artists work and provide printing and shipping services. In return for the work marketing and distributing the artists work the gallery owner charges a commission on each sale. which can be anywher from 10% to 80% of the sale price depending on who you deal with.

Naturally when selecting a gallery to display and distribute your work you should consider the quality produced by the gallery first as whatever quality ships from the gallery will reflect well or poorl on you.
Print programs like those at deviantArt are useful for checking the market but unless you're willing to buy a Premium Print subscription this is not really worth your while.
If you're serious about displaying Monetizing your art your best bet is always going to be with lower volume higher quality galleries than community galleries. Smaller galleries are generally more selective but as a result are able to charge higher prices which equate to higher commission cheques. If your work is good enough you'll find your best opportunities will lie in these niche galleries that cater to a smaller market.
These galleries are great sources of backlinks as well. Where ever possible try and get them to add a link to your website. Search engines love good contextual links. When dealing with online galleries it's extremely important to find out what rates you could get on commissioned work. You may find that you do most of your work through a particular gallery and as a result you are now growing your number of commissioned pieces. Try and find out at the outset if the gallery will be able to accomodate you with your commissioned pieces by way of shipping and printing. This is great business as producing one off pieces in your personal capacity can break the bank and send potential clients running.

As with all things in the art world spend some time trolling the net hanging about in forums and you'll soon have list of links as long as your arm to go and hit up for their services. Just remember to weigh up the offers based on what goes in your pocket not what the customer coughs up.

Some Links to get you going:
deviantART Prints
Untapped source art gallery and print store
Digital Art Print Gallery
Spacescapes Gallery