We are calling the Moon a crescent when it is smaller than a quarter of its maximal size. Under the condition, it is larger than a quarter it is called gibbous. It starts waxing when it is getting bigger and pass from a novice one to a full. Logically, it is a waning moon when it is getting smaller and passes from a full one to a novice.
For instance, when the Moon was a waxing crescent, it would be going on growing in its sizes the next day. Thus, it would steadily approach its first quarter. Afterward, there would come a waxing gibbous. It would be growing in its size until it would reach the fullness. The next phase is its diminishing. It starts shrinking and passes into a waning phase, reaching the third quarter. At would become invisible after reaching the final phase, which we call a New Moon. This is a common and natural alteration of moon's shapes.
Under the condition, you would not be capable of memorizing all these things, you can use different handy ways that would help you to find out whether the Moon is getting bigger or is shrinking. It is quite simple. Just take a look at the form of this body. It would be a crescent Moon if its shape would remind the letter "C". The letter "D" means that it is getting bigger. When it is a bit bigger again, it is a gibbous Moon. The curved form, which reminds "C" is the gibbous Moon that gets shrunken. You can also look from the right to the left to find out whether it grows or diminishes.