This is a huge carnivorous spider, and an invention of JKR. The name derives from the Latin "acro", meaning "highest" or "extreme", along with "-antula" from the word "tarantula", a large spider. Therefore the name may be interpreted as "extreme spider".
The Acromantula is covered in thick black hair and can grow up to 15 feet in size [FB]. It was created by wizards, and probably intended as a guard creature. They are, however, untrainable and extremely dangerous. They have near-human intelligence and are capable of speech, but this doesn't mean you can reason with them. Hagrid had an Acromantula called Aragog as a pet while he was in his third year at Hogwarts. It was suspected to be the creature from the Chamber of Secrets (which it wasn't) and Hagrid was expelled from school. Aragog escaped and set up what is now a well-populated colony in the Forbidden Forest [COS15]. Aragog is now deceased, but the colony continues to thrive [HBP22].
Lord Voldemort somehow convinced the remaining Acromantula to join the Death Eaters in their attack on Hogwarts, although it is not made clear how he did so. It seems fair to assume it involved an offer of letting them eat a lot of fallen enemies, however. They scaled the walls of the castle in search of prey: on one occasion Harry held them back with a Stunning spell, but it seems unlikely that it kept them away for long [DH32]. Voldemort later used their deserted nest as his base within the Forbidden Forest as he waited for Harry to come and give himself up [DH34].