Mutually exclusive projects are those projects in which acceptance of one project automatically excludes other projects. In another word, mutually exclusive projects are the set of projects in which decision to undertake one project excludes all other projects from consideration. Suppose A, B and C are mutually exclusive projects. Here undertaking project A automatically excludes project B and C. Similarly, A and C will be excluded if project B is undertaken.
For example, you have budget of $1000 to buy a computer. Suppose A, B, C ,D and E are the different models and brands of computer that are under your budget. You can buy any one computer from these available brands and models. If you decide to buy A then all other computers will be automatically excluded.