A Call sign is a very important part of being an amateur radio operator. It is your unique identifier to any other radio operator that shows that you have passed the exam and hold a license to build and experiment with radios on the amateur allocated frequencies. Once you have a valid license, your call sign is unique from anyone else in the world!

This section is for American hams. Other countries use different call signs.
All American callsigns start with K, N, W, or A.
They typically follow a format of:
2×3 (KI7IGV)
1×3 (N7CTB)
2×2 (AG7CT) – Advanced/Extra Only
2×1 (WC7L) – Extra Only
1×2 (K7RX) – Extra Only

Most callsigns have a single number in them. There are some countries that have multiple numbers in their callsigns, but these are uncommon.

You can only get a callsign by holding an amateur radio license. And you can only get a license by passing the amateur exam and paying the appropriate fees to the FCC.

In the past, the FCC enforced the callsign districts. If you lived in California but moved to Nevada, you had to request for a new callsign to match your new district.
This rule has been relaxed. When you get your initial callsign after passing the exam, you will get the number from that district. After that, you can request and get a callsign with any number, regardless of where you live.