FS - Field Separator in UNIX

FS - specify the field separator in your input file, FS can be single character or a regular expression.

Example : Change your /etc/passwd from colon separated to tab separated

awk 'BEGIN{FS=":"}{print $1"\t"$2;}' /etc/passwd


Output : (Can vary depending upon OS and permissions)
rootx
daemonx
binx
sysx
syncx
mailx
proxyx
www-datax
backupx
operatorx
haldaemonx
dbusx
nobodyx
sshdx