Mid-wicket is a run-saving position on the on-side, between 45-60 degrees from the bat. It is a defensive place to field, so you might expect to be chasing a lot of balls – but there could be a chance of a top edge coming straight for you.
Short mid-wicket is obviously a more attacking placement, but deep midwicket is used primarily to cut off boundaries. Someone fielding at deep mid-wicket could be referred to as a ‘sweeper’, as they are expected to patrol the entire boundary in that area.
The position between deep mid-wicket and deep long-on is known informally as ‘cow corner’. This term comes from people suggesting that cows could happily graze on the outfield there because the ball would never hit them. However, with modern cricket, balls are more likely to be hit towards cow corner when batsmen are heaving for fours and sixes, so don’t be surprised to see a fielder there during one-day matches!