a semiquaver is half of a quaver, which is half of a crotchet, which is half of a minim, which is half of a semibreve, which is half of a breve. Therefore, a crotchet is 1/4 of a semibreve, a quaver is 1/8 of a semibreve, and a semiquaver is 1/16 of a semibreve.
The only place I know of where this notation can still be found is in a music history textbook.
You need to get yourself a good Music Thoery book. Just be aware that a lot of the music theory out there is based around the American System of whole notes and half notes.
