Wireless USB Adapter?

Hey guys, I'm just wondering what exactly a Wireless USB Adapter does. Is it for computers without wireless cards, or is it for extending the range of a network?

I moved my Wii downstairs and it's now experiencing extremely slow internet speeds. I'm assuming it's not getting a strong enough signal from my router that's on another level. Is there a way to boost the signal without adding another wired access point?

I'm a little new to this so please keep terms used at a beginner level =)

Thanks.

Edit: Here's the particular one I'm looking at.. There's "Range Booster" in the name so this is all kind of confusing.. Would I be plugging this into my Laptop which would have to be downstairs? or the computer that's directly connected to the router? or would I be plugging this into the Wii itself? (http://www.bestbuy.ca/catalog/proddetail.asp?logon=&langid=EN&sku_id=0926INGFS10080888&catid=21120)

That item you linked is for a computer without built in wireless. "RangeBooster" is just D-Link's brand name for their proprietary technology that increases the range of 802.11g. It won't help your Wii.

What brand/model is your router? If it's Apple, then the easiest solution is to get an Airport Express and use it to extend your network. If it's another brand then it gets trickier. If you're not technically savvy then your best bet would be to get a range extender from the same brand as your router that claims to be compatible.

Thanks for the reply. I'm using a D-Link D-524 I believe, and after I read your post I just picked up a D-Link DWL-G710 Wireless Repeater for who are curious. It took about 10 minutes to set up and worked like charm.