Buy a deck of 3X5 index cards. The amount of information you need to learn or memorize will dictate which style of index card you select. For example, if you know you are going to need to memorize a lot of definitions, then index cards that have a blank front and a lined back will help you out. If you just need to write key words on the cards, then cards with two blank sides will do. You also can buy index cards in different colors to represent different academic subjects.
Write keywords on one side of the index card with a marker or pen. Flip over the card and write the definition. You also can format your cards in a question/answer system. Write a question on the front of the card. Then flip over the card and write the answer.
Place the cards in a deck and knock them on a table to align them. Pick up a few at a time--perhaps five--and punch a hole in the corner of the cards with a hand-held hole puncher.
Slip a binder ring through the holes. You now have a portable system that you can take with you most places you go. You also can place the cards in an index storage box--available at your local office supply store--to keep them in order and in one place.