This command searches for all occurrences of the text string 'fred' within the "/etc/passwd" file. It will find and print (on the screen) all of the lines in this file that contain the text string 'fred', including lines that contain usernames like "fred" - and also "alfred".

grep '^fred' /etc/passwd

This command searches for all occurrences of the text string 'fred' within the "/etc/passwd" file, but also requires that the "f" in the name "fred" be in the first column of each record (that's what the caret character tells grep). Using this more-advanced search, a user named "alfred" would not be matched, because the letter "a" will be in the first column.

grep 'joe' *

This command searches for all occurrences of the text string 'joe' within all files of the current directory.