cat and grep are essential Linux keywords. They're up there with ls and cd--you just have to know what they do.
cat basically prints out the contents of a file. So, with the file in this Gist, "some_file", if you were to type the command, cat some_file, you would get the output as the following:
$ cat some_file
Hey I'm some file.
Bob
Bob Joe Barny
Unix
Linux
La dee dah.
grep searches a file. So if you run grep PATTERN some_file:
$ grep "Joe" some_file
Bob Joe Barny
grep is case-sensitive:
$ grep "hey" some_file
hey Linux
$ grep -i "hey" some_file
Hey I'm some file.
Hey Unix
hey Linux
Add an option for line numbers:
$ grep -in "hey" some_file
1:Hey I'm some file.
7:Hey Unix
9:hey Linux
When you learn about pipes, you can filter output from one program and search for certain items, like one type of file:
$ ls /bin | grep cat
bzcat
cat
netcat
zcat
$ ls /etc/ |grep pass
mustchangepassword
passwd
passwd-
passwd.homedirs
passwd.homedirs-