Why does Terminal Emulator for Android has completely different commands comparing to GNU/Linux? I installed Terminal Emulator for Android hoping I could easily get root access,install apps via terminal etc.So,thinking Android is a Linux distribution,I felt that it must have a terminal emulator (to really feel I'm using Linux),but when I tried some commands,none of them worked!Why?
What commands are used on Android?
Android is based on the Linux kernel, but as GiantTree suggests, it is not a GNU/Linux distribution like Ubuntu. There are some commands in common (cd, cat), but some really obvious ones are not there (cp for example). As to why I suppose the main reason is that Android is not trying to be Unix-like so the common Unix commands are not needed by most applications.

If you are interested in getting more command line tools which will be familiar to you from your GNU/Linux distribution you have a variety of options. On the Google Play look for "Busybox (No Root)" or "GnuRoot". The former will give you some familiar commands (cp for example), the latter a choice of full command-line GNU/Linux distros, without the need to root.

If you do root your phone you have even more options, but just having the terminal emulator app installed will not help you to root your phone in itself. How to root your phone depends on your exact model of phone, and goes beyond your query here.

I'm sorry if this is bad news for you. I sympathize as I was just as surprised as you a year ago when I made the move from GNU/Linux to Android.