Linux Problems

Hi, I have been trying sendmail and Exim for whole day. Looking what exactly they are and how to use them. But still stuck on what they are. I found Sendmail and Exim both are MTA (Mail Transfer agent), but still my Cpanel installation shows that I have sendmail and exim install on same server, How can this be? If I have sendmail listening on Port 25 than where exim is listening for email? Anyone any idea please post comment so that I can work on my project. thanks everyone Sumit Gupta