Map/Reduce is broken into four phases 1)

getInputData

is for retrieving all the data you're going to process (e.g. find all the Invoices due today) 2)

map

(optional) is for grouping that data by some relevant dimension (e.g. group the Invoices by Customer) 3)

reduce

is for the actual processing of the data (e.g. send an email to the Customer reminding them their Invoices are due) 4)

summarize

(optional) contains statistics about the processing and let's you take action after all processing has been complete (e.g. send an email report to the Account Manager)