Both use a barre. The barre is literally a bar: it's a horizontal wood or metal barre that is held on to during both barre and ballet classes. Both barre class and ballet class involve many isometric contractions. (Think relevés or rising to your toes up and down repetitively). Both promote good posture, alignment and an engaged core.
That's about it.
While barre fitness has its roots in ballet (it was invented by an injured ballerina when she had the idea to combine ballet workouts with rehabilitative therapy), it is very different, and quite frankly, much more approachable if you're a newbie to the boutique-studio type workout.
Though fitted clothing is recommended (so that instructors can be sure you have the correct alignment and body positioning) in a barre class, it is rarely required. In some ballet studios, a very strict rule of a tight bun, black leotard and pink tights are required. Many adult or beginner ballet classes are more relaxed though, with even yoga pants allowed but ballet shoes are always a must. Check the studio's website for details or call to find out to avoid a studio faux pas.
Many barre studios allow bare feet, but some require special grippy socks. Know before you go so that you don't have to miss out on class, or fork over another $20 on the spot. There's so many cute versions now that are a far cry from the hospital-issued grip socks or toddler grippy socks that may come to mind. Check out my favorite pair: these are cute and functional. They are great for yoga, barre and Pilates but not ballet. Repeat: grippy socks will not suffice in ballet class.
You'll never see a yoga mat in traditional ballet classes, but they are a part of almost every single barre class I've been to (or taught). If not at the beginning, definitely the last part of the class will be spent on the mat, targeting the core, booty and finally (ahhh) the cool-down stretching.
In ballet, you're most likely to end class in quick jumps, pouring sweat and shaking as you give the teacher your final appreciative reverance, or "curtsy". (Does it help to know French to take ballet? Yes. Do you have to? No, but you do have to memorize the most common terms in ballet to get along in a typical ballet class.
Either from my childhood/teen years of ballet classes or my need for a punishing workout, I prefer to end a fitness class sweaty and shaking. Make no mistake; some barre studios get you good. I've found that the more easygoing, all ages, all levels relaxed studios tend to have easier classes. I tend to go to the more "hardcore" ones, but that's just me and my need to feel that I got my "money's worth". For me though, ballet is the harder and more rewarding class with a more noticeable difference in my body, but both workouts claim to (and if you put in the work, do) yield a leaner, more toned and flexible body.
In barre class, you'll never do turns or jumps or wear pointe shoes. So you don't have to worry as much if you don't have great balance: the option to hold onto the barre is always there, whereas in ballet you leave the barre for the last part of class to do the aptly-named "center" dancing. (Or "center work"). Most intimidatingly, you may have to preform combinations or choreography in small groups or *gulp* solo. (My biggest fear in ballet class).
In barre, you'll never be asked to perform a move with everyone staring at you, so relax. As in ballet, most people will be watching themselves in the mirror and the teacher will be the one to "fix" or "adjust" your positioning either with verbal cues or hands-on adjustments. The teacher usually does the moves with you, or at least describes them very well. Instructors in beginner ballet may do the same, but once you get past beginning ballet, you need to learn the names of the moves, because some teachers only recite a string of instructions quickly, usually only referring to the French names, and you'll be expected to perform them without seeing them demo'ed first.
You won't need to memorize anything for barre class, and sometimes fun accessories (or tortuous, depending on the moves/teacher) are included, like exercise bands for resistance, or squishy barre balls to help target those inner thighs or to protect your back in certain core moves.
You can do both classes at home via online videos, DVDs or YouTube workouts. You can buy stand-up barres for home practice, use a countertop (or DIY a quality barre or say, attach an IKEA curtain rod to the wall). But chances are, there's a studio near you that offers ballet or barre classes.
*Important: never pop on a pair of pointe shoes unless teachers have given you the approval/you've appropriately strengthened your feet and ankles. Otherwise, you're just asking for an injury. Speaking of injuries, no matter what fitness class you try out, let the teacher know about any past or current injuries, introduce yourself and tell them it's your first time.
Give barre or ballet a shot, and check out my tips on never letting any workout class intimidate you if you're hesitant. Don't be; everyone in the class was a beginner once.
See you at the barre!