It is a top setting onion. Two types generally available, Egyptian and and Catawissa. I keep a small bed of Egyptian, mostly a curiosity. They are perennial. They never form much of a bulb. Small sets form where the seedheads of a regular onion would be. These get about the size of pearl onions and can be used in soups etc. When these stems fall over the sets take root and grow wherever they fall, hence " walking onion". They are very pungent, even the young ones used as "green" onions".

Here is a link that might be useful: Walking Onion

Try to do a search on this forum for egyptian or catawissa onions and there will be a few pictures that will show you what you're getting into. Actually, they are neither the big ones nor the green bunching variety ... they are perennial onions that put out a bunch of small onions instead of a flower at the top as well as another set of small onions around the base of the plant. It's only my first year trying them but they seem to like a 6-8" spacing and the egyptian ones grow a really interesting medusa head that is starting to yield some 1/2" onions that will be good for pickling.