Many names for the same thing.
So, what is it and how can you fix it?
Well the short answer is it depends.
It depends on the dog; their age, breed (or mixes of breeds), history, prior training, social experiences and genetics.
It depends on the owner; their level of confidence, personal knowledge of training & dog psychology and how to effectively reward and correct their dogs.
Reactivity can generally be broken into a few groups. Genetics, Environment & Social.
Genetic reactivity is probably one of the toughest to truly "fix" because the clay of the dog you have to mold, to shape into the dog you want is flawed. This takes time, love, patience and a lot of support and management on the owner's part so the dog can overcome the difficulties they have. Very often these dogs can gain a higher quality of life but will never truly be "social butterflies" and will need the support and knowledge of their owner to help them stay out of situations where they're put in a defensive or fearful position.
Environmental reactivity is when the dog hasn't had the opportunity to be confident and comfortable in a variety of places with different levels of crowds, people, dogs and kids. Puppies or dogs that weren't socialized or who weren't exposed to normal life during the critical developmental stages can show fear at things we would take for granted. With an empathic and patient program generally these dogs are much happier and confident once they learn the world isn't' such a scary place.
Social reactivity is when the dog doesn't have the skills or confidence to interact appropriately with other dogs and people. Generally this relates in some way back to the two above mentioned issues, genetics and environment. Socially awkward dogs can inadvertently cause dogs fights, scare other dogs or people and knock over less mobile adults' and children. Often they're exuberant teenagers that simply don't know their own strength and have a tendency to be a bull in china shop. These dogs haven't learned social skills or emotional frustration tolerance so they understand how to mange their frustration levels, drives and desires in a productive and appropriate way.
Reactivity can come in many forms and can be very mild to severe, showing up as minor issue or severe aggression and fear. Taking care to get it addressed quickly and effectively by a trainer is your best bet to help your dog have a long happy life.