A longarm generally has a head that is 18" or greater, and the machine moves over a frame. There are also shortarms and midarms that move over a frame. Some people sit at their longarms too. There are lots of options for you - one is machine quilting in sections with your current machine. (See the book Machine Quilting in Sections by Marti Michell.) I did that for a few years before buying a longarm, and I found it was much easier than trying to layer and quilt a large quilt in my sewing machine. The best thing is to try out all kinds of machines and decide which type is best for you. If you can go to a machine quilting show, there are usually lots of vendors with all the different types of quilting machines.