format - Format a string in the style of sprintf
format formatString ?arg arg ...?
This command generates a formatted string in the same way as the ANSI C sprintf procedure (it uses sprintf in its implementation). FormatString indicates how to format the result, using % conversion specifiers as in sprintf, and the additional arguments, if any, provide values to be substituted into the result. The return value from format is the formatted string.
More at: http://purl.org/tcl/home/man/tcl8.4/TclCmd/format.htm
Tips and Tricks with format
You can use [format] to produce unsigned integers for display (but don't reckon with them - for expr they're still signed!):
% format %u -1 4294967295
See floating point formats for discussion on how to write format strings to handle floats...
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command