The crotchet is the most common style of musical note. It is also the length of note that ever other style of note is counted from (e.g., a semibreve is four crotchets). It is denoted by a small, flat circle that is filled with black and has a long straight line protruding upwards or downwards from it. It is also known as a quarter note because it counts for one beat in a 4-beat bar.
The semibreve is also known as a whole note and counts for four crotchets. In a four-beat bar it is placed at the beginning because it counts for the whole musical bar. It looks like an oval and has a blackened edge that is thicker on the left- and right-hand sides than it is on top and bottom; essentially it looks like a flattened, bold "O."
A minim is the style of note that represents two crotchets or half a semibreve. It is styled exactly the same as a crotchet but has a hollow, white oval instead of a black one. In a four-beat bar, the minim can only be placed in the first, second or third spot because it lasts for two beats of the bar and cannot be played over or into a new bar.
A quaver is considered half a crotchet (eight in a bar), and a semiquaver represents a quarter of a crotchet (16 semi-quavers in a bar). A quaver is styled in exactly the same way as a crotchet but has a flicked tail dropping from the long straight line. Similarly, a semiquaver looks like a crotchet as well but has two flicked tails dropping from the line.
A dotted note can be applied to any of the above styles of note. It is represented by a small black dot that appears on the right-hand side of the oval shape of the note. What it means is that you must play the length of the normal note plus a half. For example, a dotted minim would last three beats of the bar (its normal two beats plus one), and a dotted semibreve should be played for six beats.