Some research facilities will do things like assign your blood a bar code with a number. This is placed on your blood, rather than your name, to ensure privacy.
Does it hurt?
It depends on what you're being asked to do. Often times, all you're doing is giving blood. If that's the case, all you'll feel is a needle stick.
What are the risk factors?
If you're just giving blood, there isn't much of a risk. If you're concerned about the risk factors related to the research you're participating in, ask questions. It's important that you're comfortable with what you're volunteering to do.
What's the recovery time?
After a few hours and a good meal, your blood will probably be back in your system, and you will be back to normal.
Are there any follow-ups?
It will depend on the research. For some research, a scientist might need you to come back for more data, but often times, they only need you once.
Source: Ginger Roberts, a certified clinical research professional and clinical coordinator at the Oklahoma Medical Research Foundation; the U.S. Department of Health and Human Services.