A crotchet or quarter note is a note played for one fourth of a whole note (or semibreve). Quarter notes are written as a filled-in oval (or round) note head and a straight, flagless stem (the line). The stem usually points up (like a d) if it is below the middle line of the stave or down (like a p) if it is on or above the middle line.
When beginners start to learn to play an instrument they are often taught that one crotchet lasts for one beat (one count). That is because they are learning pieces with simple time signatures. It is, however, important to remember that other note lengths (minims, dotted crotchets etc.) can also be treated as one beat, depending on the time signature.