Difference between ls -l , ls -ltr and ll?

First of all, the mentioned commands do not generate the same output. Here are the details:

1. ll : There is no command like ll, in Ubuntu it is an alias for the command ls -alF. you can find it by:

   $ type ll
   ll is aliased to `ls -alF'
   

   • -a option is to show hidden files (will show . and .. too)

   • -l option is to show the output as a long list along with various attributes e.g. permissions, file sizes, use, group, modification time etc.

   • -F will append one of */=>@| to the entries, it is basically used to differentiate files from directories as it will append / to the directory entries

   Example :

   $ ll
   total 24
   drwxrwxr-x 4 chayan chayan 4096 Jun 25 13:53 ./
   drwxrwxr-x 3 chayan chayan 4096 Jun 25 13:34 ../
   drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data/
   -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
   

2. ls -l : As mentioned earlier -l will show the entries as a long list along with various attributes.

   Example:

   $ ls -l
   total 16
   drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data
   -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
   

3. ls -ltr :

   • -t option will sort the entries by modification date (with newest first)

   • -r will reverse the sorting order.

   As -t will sort by modification time with newest first, -r will cause the reverse i.e. oldest entries will be shown first now.

   Example (Adding a file and a directory to make it clearer) :

   $ ls -lt
   total 16
   drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:50 log
   -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
   -rw-rw-r-- 1 chayan chayan  208 Jun 25 13:49 move.sh
   drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data
   
   $ ls -ltr
   total 16
   drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:35 data
   -rw-rw-r-- 1 chayan chayan  208 Jun 25 13:49 move.sh
   -rw-rw-r-- 1 chayan chayan   88 Jun 25 13:50 filenames.txt
   drwxrwxr-x 2 chayan chayan 4096 Jun 25 13:50 log
   

Check man ls to get more idea.