You can wash your comforter (even if its a feather comforter) in a washing machine but you need to use one of those oversized machines that some laundromats have.
To wash your comforter, get one of those large machines and use a gentle detergent. (Its better to use less detergent than too much). Make sure you have it on delicate cycle.
When you dry your comforter, put it on a low heat setting. You may want to throw a couple of clean tennis balls in the machine to help 'fluff' it up while its drying. If you don't have any clean tennis balls, I like to take the comforter out once in a while and shake it up before tossing it back in the dryer.
You can usually wash your comforter in a washing machine. If you have a large capacity washer you can wash it at home. If you don't have one, then you need to use a front loading washer at your local laundromat. Use a gentle detergent such as Woolite and make sure it is on the gentle cycle. When you dry it, use the delicate or low cycle on the dryer and do not over dry it. I like to hang mine over the shower curtain rod after it is nearly dry. You should shake it out to fluff it up several times before putting it back on your bed.
Depends on the size of your comforter and the size/type of your washing machine. Large comforter in small top loading washer = not ideal. You want to make sure there's some room for the comforter to move about in the machine. Most new machines should be able to handle it. If your washer is 2 cubic feet or more, you should be fine.
I have had good & bad experiences washing comforters in the machine. If it's just cotton & the tag says washing machine, go for it. If its down or something special you would be very upset over if it were to be ruined I would have it dry cleaned. Better safe than sorry!