If you own a table saw you know how important it is to keep your hands away from the blade at all times. One tool that helps is a push block. I prefer a push block to a push stick because I feel like I have better control of the wood. The block has more contact with the wood and gives me more down force to hold the wood to the table.
For the highest degree of control and safety you need three points of pressure on the wood. You need forward pressure to push the wood into the blade. You need downward pressure to hold the wood to the table and you need side pressure to push the wood against the rip fence.
A push stick only gives you forward pressure. A push block or push shoe as some call it gives you forward and downward pressure. This still leaves the need for side pressure. In some cases you can safely use your off hand but only if you can keep your fingers a full hands width from the blade. If you cannot keep this safe distance then you should use a feather board.