I got an email about a purchase I didn't make

You are sure that it was a genuine email from Apple and not a phishing attempt trying to get your account and payment details ?

Phishing and similar emails : Phishing & Other Suspicious Emails

Phishing emails : Identifying fraudulent "phishing" email - Apple Support

Genuine iTunes emails : Identifying legitimate emails from the iTunes Store - Apple Support

Viewing your account's purchase history : See your purchase history in the iTunes Store - Apple Support (on an iOS device you can view the last 90 days purchases via http://reportaproblem.apple.com)

If it's a genuine email and it shows on your account's purchase history and you didn't make it, and nobody else has access to your account or devices/computers and did it, then contact iTunes Support : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption . And change your account's password

If it is a phishing email then forward it to Apple : reportphishing@apple.com , and then delete it.

And it showed on your account's purchase history ? Email addresses can be spoofed

iTunes and Apple ID are the same thing, they are just different ways of referring to an account - and each account has to have a unique primary email address.

The email didn't have any links in it e.g. phishing emails usually have things such as 'click here if you didn't do the purchase in order to ask for a refund'. And, depending upon the type of item, does it show in the Purchased link under Quick Links on the right-hand side of the iTunes store homepage on your computer's iTunes as being redownloadable (audiobooks and tones are one-time only downloads so won't show, neither will in-app purchases) ?