I know this post is really old but someone quoted it to me recently so I wanted to update it for accuracy and help anyone else searching for this question as it comes up in google search. This is something that a lot of people miss use. It's really good to have a clear understanding on. I've seen CCIE's get this wrong and cause major miss-understandings.
Here's a quote from the Cisco Press book:
A site with a single ISP connection is single-homed. This is fine for a site that does not depend heavily on Internet or WAN connectivity. Either use static routes, or advertise the site routes to the ISP and receive a default route from the ISP.
A dual-homed site has two connections to the same ISP, either from one router or two routers. One link might be primary and the other backup, or the site might load balance over both links. Either static or dynamic routing would work in this case.
Multihoming means connecting to more than one ISP at the same time. It is done for redundancy and backup if one ISP fails, and for better performance if one ISP provides a better path to frequently used networks. This also gives you an ISP-independent solution. BGP is typically used with multihomed connections.
You can take multihoming a step further and be dual-multihomed, with two connections to multiple ISPs. This gives the most redundancy. BGP is used with the ISPs and can be used internally also.