Lol MDMA is ecstasy, E, M, X, XTC, Molly, Mandy, and Moonrocks. Other MDxx substances are included in the street slang such as MDA, bk-MDMA, MDE, MDEA, MDAI, MDMAI, and so on. However, the most common one you'll find of these MDxx substances is MDMA. It's an empathogen that causes a large release of seratonin to flood your brain, which in turn sets you on a course for emotional bliss and love and heightened tactile sensation. It puts you in a state of....ecstasy?
2C-B is a psychadelic which causes physical/mental/visual effects that are a night and day difference from MDxx substances. A "heavy" body high where the buzz throughout feels like you're packing an extra 100lbs on your body. It causes LSD-like visuals such as swirling and oozing designs of darker colors when contrasted to a lighter background. Paint texture and color differences on walls, texture on counter tops, daytime clouds against a blue sky, trees moving like there's a breeze on a calm day, etc. It reacts on different receptors in the brain than MDxx substances, therefore giving completely different effects. There is a euphoria that accompanies it, but not the overwhelming "love" that MDMA provides as it does not "bust the seratonin dam" in your head.
There's the layman differences in it. I'm sure the scientific side is soon to follow from other members. You can easily differentiate between the two chemicals by a simple and inexpensive route. A test kit. dancesafe.org sells them and the color reactions that each of these chemicals give are as blatantly obvious as their effects they have on you.
Not only that, but depending on where you live can completely change your odds of actually getting the real chemical themselves. I mean no belittlement in saying this, but if you are asking questions such as the title of this thread, then your experience/knowledge leads you blindly right into the cross-hairs of taking some really horrible chemicals sold to you as MDMA or 2C-B or whatever else you THINK or are being TOLD you're buying.
The world is flooded with garbage and money hungry people that could give two squirts of piss about you and if what they're selling you could hurt you. Research, research, RESEARCH everything you can possibly find that you will choose to ingest and ALL of their effects on you, how they work to cause those effects, what not to do/take/be on when you choose to take them, how you can assure yourself that what you're taking is what it it's supposed to be, and proper dosing of each because a standard dose of what you THINK to be MDMA at 100-125mg could horribly wreck your fuckin shit sideways if it turned out to be 2C-B who's "sweet spot" dose is 20mg.
Those are astronomical differences when you're confusing two chemicals such as these. Research my friend.