Homo erectus evolved out of
Homo habilis somewhere around 1.78
MYA (Million Years Ago), and lasted until about 600,000 years ago without major changes. (This is an impressively long time.)
Usually any Homo with a brain larger than H. habilis and smaller than Homo sapiens is classified as H. erectus (I am oversimplifying, but in practice, that's how it works out). However, some people use another category, H. ergaster, for some fossils that seem more like modern humans than do most H. erectus remains.
Homo ergaster means "Work Man", referring to their use of tools. H. erectus also used tools, as did other early hominids. (At the time that H. erectus remains were discovered, it was believed that it had been the first primate to walk upright. It was not, but the name remains.) But the name 'ergaster' was there to be used, and they needed to call it something. Homo ergaster is identified with theb Achulean tool type.
Even though H. ergaster has traits that make it seem more closely related to H. sapiens, it did not evolve from H. erectus. There is always debate about dates, but H. erectus lived from about 1.78 MYA to 0.6 MYA, while fossils of H. ergaster have been found from about 1.78 MYA to 1.2 MYA.
So, what's the difference? H. ergaster had a higher cranial vault and a thinner cranial bone than H. erectus, giving it a slightly larger cranial capacity. It also lacked a sagittal keel. It is suggested that H. erectus's lower skull and bone thickness may be specialized traits evolved later on. Whether these differences deserve a separate species name is a matter of debate.
The most famous example of an H. ergaster is the "Turkana Boy".
For other things splitters have done for little (if any) reason, see Homo heidelbergensis and Homo antecessor. You can never have too many homos!
Partial references (Netscape crashed):
Biological Anthropology by Michael Alan Park
http://www.uea.ac.uk/~x9706887/Hergaster.html