A double throw switch is a 3 position switch. A manual transfer switch is an example of a double throw switch.
A single throw switch has 2 positions. An example would be a light switch.
Close.
A single throw switch has only two positions and only two contacts for each pole. It will either be open in one position and closed in the other or closed in one position and open in the other. (and if all the poles are identical you can choose which just by rotating it 180 degrees>)
A common double throw switch will still have only two positions. But it will have three contacts per pole. One common pole will be connected to one of the remaining two in one position and to the other one in the other position.
A double throw, center-off, switch has three positions. Exact same contacts per pole, but in the center postition the common is not connected to either of the other contacts.
A three-way switch is double throw.
A transfer switch is double throw with lots of care taken to guarantee that it breaks one side of each pole before closing the other. And it may have a center off position for servicing or to let motor and reactive voltages die down before connecting the alternate power source.
They even make center-off switches that force you to pause in the middle instead of going directly from one side to the other. Some ways of interlocking CBs to make a transfer switch will do this because you have to manually move the interlock slider after opening the first breaker.