Is it possible to see the login history, i mean to see if someone has used the computer in my absence and when he used it .
if it is possible, where can i get the log ?
if not, is there a program that logs all the logins and their time ?
Well appending your question and oli's answer if you are on laptop then you can also check it through grepping the exact content like
sudo cat /var/log/auth.log | grep "Lid opened"
sudo cat /var/log/auth.log | grep "Lid closed"
and whether he/she perform any kind of activity through sudo permission by
sudo cat /var/log/auth.log | grep "session opened for user root"
sudo cat /var/log/auth.log | grep "session closed for user root"
It will give you extra information what you want to know about user's logged in your system without your permission :):)