Windows as well as other platforms use two different forms to display a text-cursor. One, which is used most, is the bar-formed cursor which rests between two letters. The other has block form and rests on a letter, replacing it if new input is done and going one letter further. This is annoying if you activate it by accident.
You activate as well as deactivate it with the Ins key on usual keyboards. If you are using different hardware, look for a key combination equivalent to the Insert-key.