Lizardmen are a race of reptilian humanoids ranging from small and quick to gigantic and slow, they are a hostile army found in Warhammer Fantasy and are an ancient but extremely hateful species most at home in vast jungles and other exotic places.
Lizardmen's status as villains was showcased in how they were supplied in some of the Warhammer Fantasy boxed games as the opposing army to humans, replacing Orcs in that regard (though different box sets have had different armies showcased - the more recent having Dark Elf and Skaven for example).
Despite being shown as villains, they are, in fact, servants of the Old Ones- a group of benevolent gods who created the Warhammer world. That being said, their ideals for order are far removed from the ideals of other Order-aligned nations. Their intent is based on a series of crystallized blueprints made from the Old Ones' own thoughts. Unfortunately, the blueprints in question are somewhat metaphorical in meaning, so understanding them incorrectly can lead to disastrous consequences.