You see these all the time in movies, usually when some poor guy has been wrongly imprisoned and begins counting the days since his incarceration, but what are these markings called? I used to know, but I've forgotten and now it's really bugging me.
They are called tally marks.
Tally marks, or hash marks, are a unary numeral system. They are a form of numeral used for counting. They allow updating written intermediate results without erasing or discarding anything written down. However, because of the length of large numbers, tallies are not commonly used for static text.