Use on user pages or user talk pages when people shouldn't expect them to be answered quickly. When using simply {{busy}}, the template produces the message above

There are two unnamed parameters. The first which replaces "this user" with the name placed.
IE {{busy|He}} produces

He is busy in real life and may not respond swiftly to queries.

The second unnamed parameter replaces "in real life" with whatever is put. Unfortunately, at the time being it requires for there to be something in the first parameter as otherwise nothing will be shown for the first parameter (DO NOT use {{busy||foo}}).
IE {{busy|This user|at [[school]]}} produces

This user is busy at school and may not respond swiftly to queries.

• reason, use to specify a reason why you will be busy.
  

IE {{busy|reason=not being allowed to edit Wikipedia while at work,}} produces

This user is busy in real life due to not being allowed to edit Wikipedia while at work, and may not respond swiftly to queries.

• end, use to specify until when you will be busy.
  

IE {{busy|end=Monday evening (UTC)}} produces

This user is busy in real life until Monday evening (UTC) and may not respond swiftly to queries.

• msg appends any custom message at the end.
  

IE {{busy|msg=April Fool!}} produces

This user is busy in real life and may not respond swiftly to queries. April Fool!

• image will display a different image other than the standard,
  

IE {{busy|image=Information icon4.svg}} produces

This user is busy in real life and may not respond swiftly to queries.

• small will display the small version of this box,
  

IE {{busy|small=yes}} produces

This user is busy in real life and may not respond swiftly to queries.

• descriptor, use to specify an alternative to "busy".
  

IE {{busy|descriptor=sailing their yacht}} produces

This user is sailing their yacht in real life and may not respond swiftly to queries.

{{busy|Johnny Doe|descriptor=working|at school|image=Information icon4.svg|reason=school being compulsory|end=Saturday morning (EST)|msg=I won't be on for Saturday the 25th, though.}} produces

Johnny Doe is working at school until Saturday morning (EST) due to school being compulsory and may not respond swiftly to queries. I won't be on for Saturday the 25th, though.