Welcome to Android Forums! Glad you signed up! Non-deletable apps are called "bloatware." Perhaps this is what you mean. These are apps that are included in your carrier's software. They are generally there to increase their profits, should you use them. They have it set up that they will be there for as long as you have your phone and since they run in the background, they generally slow it down.
Unless: You modify your phone. There is a process called "rooting" your phone, which gives you access to all adminstrative functions of the phone's system. Your carrier does not want you to have these privileges, for obvious reasons. You have the freedom to do all sorts of things with your phone, if you root, including delete (or freeze) bloatware.
For more information, you can go to your phone's section and look around. PM me if you have any more questions.
Steven