What is the inactive reserves really for?

I have joined the DEP and sign the contract for 3 years 27 weeks active duty. It also says I will be in the inactive reserves for 8 years and if an emergency happens I can be called back. My question is if I complete the 3 years 27 weeks can I just retire if they don't need me back? Also after the 3 years 27 weeks is up will someone remind in the army or tell its time to reenlist or retire?