A summer analyst or SA is someone undertaking an internship within a bank, usually for 3 months over the summer after the junior year. A summer analyst typically does work similar to that of a full time analyst, although with a lot more 'grunt' work (i.e. fetching coffee, moving documents between departments etc.).
The main way for graduates to get work in an investment bank is through having completed an internship as a summer analyst with the bank, and then being made an offer to return full time when they have completed university.
Summer analysts have to work the same hours as normal analysts (80-100 hours a week) and are paid the same amount on a pro-rata rate (~$70k per year) but with no bonus.