Hamas was founded in 1987 as an alternative modern movement to the PLO. Hamas has a military wing and a political wing.
Hamas is the legitimate government of Gaza and won the election in 2006 by a marginal vote. However no election has since taken place even though Hamas has called for it every year. Hamas opposes the 17 year illegal siege on Gaza imposed by Israel and seen by many as a resistance group to the occupation.
Here in the UK the political wing was secretly declared a terrorist organisation by Priti patel without going through the procedures of approval. This is now being contested.
Hamas military wing has conducted operations which can be categorised as terror acts, but this depends on the definition of civillians vs occupiers on the land.