During the Secret Wars series in the 1980's, Spider-Man acquired a black suit with special powers, like enhanced speed & strength, unlimited webbing, and being able to shape-shift into any clothing. It also just looked awesome. However, it turned out that the "suit" was actually alive, a symbiotic alien life form that had merged with Peter Parker.
He tries to separate from the symbiote, using sonic waves that disrupt the alien. The alien then finds Eddie Brock, a down & out reporter who is ready to commit suicide. The symbiote bonds with Brock, and takes on his aggressive and violent personality. The merged pair becomes the terrifying creature known as Venom, one of Spider-Man's most powerful enemies and a great villain character.
Anyway, the Venom character has been done in movies before, in Spider-Man 3, but the full Venom appeared only briefly and was handled in a pretty lame way. With many years and more advanced CGI since then, this iteration of Venom looks pretty good, and hopefully the character gets more screen time.
Unfortunately, as is the situation with certain superhero movies, the involvement & ownership of these properties by different studios leads to results that do not do the characters or their stories justice. It appears that, because this is a Sony movie and not a Marvel production, Spider-Man will not be a part of this movie, which is idiotic. Venom's origin story IS Spider-Man, so to not include him would be like having a Joker movie with no Batman.
I'm pretty critical of movies in general, especially when it comes to characters that I am very familiar with & like. Sometimes, I wish I could be a consultant for these types of movies to tell everyone how things should be done. When I read other people's opinions online, most people complain about the same things I criticize. Perhaps one day there will be a proper movie with Spider-Man vs. Venom that is true to the original story. Until then, I won't bother seeing this in the theater, but may see it eventually if it seems to get good reviews.