Music notation mainly shows two different pieces of information: the pitch and the duration. I believe that you have come across the terms half note or eighth note… So, let’s go in descending order; from the longest to the shortest.

The whole note (also called a semibreve): it fits perfectly in a 4/4 bar; it’s as long as 4 quarter notes.
The half note (also called a minim): we need two half notes to fill a bar in 4/4; it’s as long as 2 quarter notes.
The quarter note (also called a crotchet): we need four quarter notes to fill a bar in 4/4; it’s as long as 2 eighth notes.
The eighth note (also called a quaver): we need eight eighth notes to fill a bar in 4/4; they usually go in pairs.
The sixteenth note (also called a semiquaver): we need 16 semiquavers to fill a bar in 4/4; they usually go in fours.
The thirty-second note (also called a demisemiquaver): we need 32 demisemiquavers to fill a bar in 4/4; they also go in fours or in eights as they are easier to read this way.
The sixty-fourth note (also called a hemidemisemiquaver, *lol*): we need 64 hemidemisemiquavers to fill a bar in 4/4; they also go in fours or in eights as they are easier to read this way; they are usually used as embellishments but let’s be honest- they only exist to help the composer troll the performers…