What - the removal of a sample(s) of blood usually from your arm for analysis. The process can be a little uncomfortable.
Why - there are many reasons why blood tests may be required and your doctor will explain which may be relevant to you.
Preparation - usually no specific preparation is required BUT occasionally when testing for fats or glucose in your blood a fasting sample may be required (e.g. a morning blood test with last food the evening before).
Risk - there is little risk associated with the removal of a blood sample but occasionally infections of the needle-site or even the bloodstream can occur.
How long will it take – taking blood only takes a minute or two, but there may be a queue!