What is a semibreve, minim, crotchet, quaver and semiquaver? 

I semibreve is what Americans call a whole note and lasts for four crotchets ( or `1/4` notes).

That means we count to four whilst holding the note.

A minim is what Americans call a half note and lasts for two crotchets ( or `1/4` notes).

That means when we play a minim we count to two whilst holding the note.

A crotchet is what Americans call a quarter note.

When we play a crotchet we count to one whilst holding the note.

A quaver is what Americans call the eighth note and lasts for half a crotchet (or `1/4` note).

That means when we play a quaver we count to a half.

A semiquaver is what Americans call a sixteenth note and lasts for a quarter of a crotchet (or `1/4` note).

That means when we play a semiquaver we count to a quarter.