I have a charge on my bank account I did not purchase

We are fellow users, so we won't know what it's for.

You've checked your account's purchase history : See your purchase history in the iTunes Store on a Mac or PC - Apple Support ?

If you don't recognise the purchase, and nobody else has access to your account or devices/computers and did it, then change your account's password (after changing it : What to do after you change your Apple ID email address or password - Apple Support) and try contacting iTunes Support : http://www.apple.com/support/itunes/contact/