You can report bugs in excel sheet in a way such that how 
you write testcases in excel sheet.

for testcase the format is TC ID,Descriptiion,data,expected 
result,actual result,pass/fail,status.

for bug reporting the format is 
Project,description,status,Detected by,Assigned to,date 
detected,test lead,severity,priority,Bud id.

once the tester found the bug means he/she entered it in 
excel sheet and send it to concerned developer.

Also BUG TRACKING TOOL's are used to report bug's..eg.test 
director,Bugzilla..