SU is the brand name, it stands for Skinners Union. As for the differances between carbs
(weber,SU or other) & carb types (downdraft, sidedraft & even the old clanger updrafts) theres a lot to explain, my post would be pages long. There's a decent article in the Tech Section thats worth checking out.

Without going int to much detail, the SU is a single throat "variable venturi" sidedraft carb, it only has one jet (main jet) in which a tapered metering pin slides up and down through, depending on air speed (throttle opening/revs) this meters your fuel.They are a great carb when set up correctly.

A Weber sideraft is a twin throat(two barrel) fixed venturi carb it has many fuel metering systems(circiuts) pump/ idle/choke enrichment/progression or transfer/main.
The right amount of air fuel mixture is regulated by a lot of jets and two emulsion tubes. these carbs can be set up for almost any engine,even the venturi's are interchangable.They are an awsome carb. Getting them set up correctly is an art, but believe me its all worth it when youve finally got them spot on.