In general, there are fillet welds and butt welds
Anyone involved in welding immediately recognises whether they are looking at a butt weld or a fillet weld. However, coming up with an infallible definition in words proves far from easy!
Here are some (attempted(!)) definitions on the Web:
I will leave it at that, because I know of no easier more obvious definition than these sites present.
In general though, in our case of structural steelwork, the situation makes it simple because welds will be either T-fillet welds between intersecting sections or butt-welds between sections meeting end-to-end.
(R. Smith, 17Dec2014)