In injection moulding, the plastic is forced into the mould at very high pressure. The mould is then kept at a given temperature while the polymer freezes, the halves are opened and the finished article is ejected. For small parts, lots of moulds are often mounted on one machine and the molten plastic is injected into them all simultaneously. Injection moulding can produce far more complex items than extrusion, but it isn't a continuous process. 

The finished articles it produces are in their final shape and can be rigid or soft, compact or cellular. Injection blow moulding allows hollow articles to be produced.

With overmoulding (co-injection), it's possible to produce articles with rigid cores (metal or polymer) by placing the cores in the mould before the plastic is injected.