Your .mac Mail account will continue to operate normally as long as you access the account from the Mac's Mail program. Read this:
What if I just want to keep using my email on all my devices?
As of May 1, you can choose to keep using your mail after MobileMe ends, even on devices that don’t meet the iCloud system requirements. Just go to me.com/move and select the option to keep using your email after MobileMe ends. Once you have completed this short process, your mail will continue to work on devices that don’t meet the iCloud requirements after MobileMe ends on June 30th, 2012.
iCloud is a collection of features that allows you to sync Safari bookmarks, Calendar items, Contacts, and iWork documents (Pages, Numbers) among multiple devices, among other things. Your photos in iPhoto will not be affected.
iCloud is most valuable for people with iOS devices and Macs and requires OS X 10.7 "Lion".
Here is some more information: http://www.apple.com/mobileme/transition.html