the process of inserting a cannula into a superficial vein in order to establish (intravenous) IV access.
sometimes, a little local anaesthetic is used to numb the area just before poking the cannula into the skin. this is more often done when a large bore needle (larger than an 18 gauge, for example) is used.
medical students stuff up cannulation pretty often, causing pain and swelling to the unfortunate patient. still, they have to learn somewhere...
see also: how to put in an intravenous cannula