A callbook is a directory of licensed operators, keyed by call sign. It answers "who is that?" in a second: name, location, license class, and often more. Once printed in thick volumes, callbooks are now searched online or built into logging software. Nothing in them is secret: license records are public.
Sources
The license record is the authority; everything else is optional, self-reported or cached.
The official licensing database. In the US, the FCC keeps the record of every license: holder, class, call sign, grant and expiry dates, and the address on file. It is the authority on whether a call sign is valid and current. Other countries keep similar registers; their detail and access differ.
Community callbook sites. Operators add a biography, photo, QSL instructions and other details. They often copy license data, so they can lag behind it.
Logging software. It queries one or more of these sources as you type a call sign, and fills in name, location and grid square for you. See Logging and contest software.
Some uses need no lookup at all: the prefix of a call sign tells you the country, and in the US the number usually tells you the region. See Call signs and what they mean.
What to do with it
Name and location for a friendlier QSO, or to check your log.
QSL information: whether and how the operator confirms contacts.
Is this call sign valid? An unfamiliar call that appears nowhere may be a mistake, a very new license or a station that should not be on the air.
Prefix lookups for country credit in DX awards.
Cautions
Information goes stale. People move, change class or let a license lapse, so a callbook shows what was on file, not where the operator is now. A portable station is not at its home address.
Self-reported data is unverified. A bio is what the person wrote.
Privacy. The address you give the licensing authority becomes part of a public record. Think about which address you provide; many operators use a mailing address (such as a post office box) where that is allowed.
Be courteous. Looking up a call sign is normal. Using what you find to turn up uninvited is not, nor is publishing someone's details.
Check each service's own policy: what is shown, what is free and how to correct your record changes over time.