What is the difference between ls and l?

SHORT ANSWER: understand what exactly this alias does, you can check out the ~/.bashrc file and search for the term "alias l=". It is nothing but ls -CF

LONG ANSWER A good way to inspect what a command is:

type l

If it's a program or a script, it will give you its location, if it is an alias, it will tell you what it's aliased to, if it's a function, it will print the funciton; otherwise, it will tell you if it is a built-in or a keyword.

Examples:

$ type l
l is aliased to `ls -CF'
$ type find
find is /usr/bin/find
$ type connecthome
connecthome is hashed (/usr/local/bin/connecthome)
$ type grep
grep is aliased to `grep --color=auto --binary-files=without-match --devices=skip'
$ type hello_se
hello_se is a function
hello_se () 
{ 
  echo 'Hello, Stack Exchangers!'
}
$ type type
type is a shell builtin
$ type for
for is a shell keyword
$ type nosuchthing
-bash: type: nosuchthing: not found

$ l --help
l: command not found

Looks like you have an alias set up in your environment. Perhaps you have inherited a .profile, .bashrc or similar containing something like alias l='ls -F'.

-F, --classify
              append indicator (one of */=>@|) to entries

Try which l and alias to track down its definition.

FIXED: l is an alias for ls -CF ( I am not really sure ) in the default .bashrc in ubuntu

You can just type alias to check out all the aliases. It would be mentioned there.