Using it
Type a message and a keyword. The rows show the plain alphabet with the cipher alphabet under it, so you can check each letter. Swap the boxes to decode. Then the rest picks how the alphabet carries on after the keyword: from A in order, which is the usual way, or from the letter after the keyword's last letter, which some puzzle books use.
How it works
Write the keyword with any repeated letters taken out, then the letters of the alphabet it didn't use. With ZEBRAS, the cipher alphabet starts ZEBRAS and goes on CDFGHIJKLMNOPQTUVWXY. Line it up under the plain alphabet and swap: A becomes Z, B becomes E, C becomes B, and so on. Decoding reads the same table the other way up.
The keyword makes the key easy to remember and to pass on: one word instead of 26 letters in a jumbled order. It's the same idea as the keyed alphabet on the Kryptos sculpture, which starts KRYPTOS.
How strong it is
Not very. However the alphabet is mixed, each letter always becomes the same other letter, so the most common cipher letter is probably E, and short words and repeated patterns give the rest away. The first known description of that attack, frequency analysis, was written by al-Kindi in the 9th century. The substitution cipher solver on this site does it automatically, without the keyword.
Questions
What if the keyword has repeated letters?
Keep only the first of each: HELLO becomes HELO, then the alphabet carries on from A without H, E, L and O.
How do I decode a keyword cipher without the keyword?
Treat it as any substitution cipher: count letters, look at short words, or paste it into the substitution cipher solver.
Is this the same as the Vigenère cipher?
No. Vigenère uses its keyword to change the shift for each letter, so one letter can be written many ways. A keyword cipher uses one fixed alphabet throughout.
Sources
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