As
eric+ says, ctime is short for change time, or st_ctime from the
stat structure. What he doesn't say, is that it is
attribute change time, not file content change--that's
mtime.
Any system call that modifies a file's inode will update the ctime.
This does include chmod and utime but not read or write.
The -c option of the ls command causes ls to use the ctime (instead of the default of mtime) when sorting by (-t) or displaying (-l) the time.
Note that of the three times listed in the stat structure,
ctime is the
only one that can't be set by a standard system call. (You can change it
to the current time, but you can't set it.)
The ctime libc function returns a character representation of a given unix time. Not only does the ctime function use a static buffer, but it also
uses a static format. For more flexible time formatting, try strftime which uses a printf style format string, and since you supply the buffer, it is even thread safe!