OpenERP's tax system runs around three major concepts:

Tax Code (or Tax Case), used for tax reporting, can be set up in a hierarchical structure so that multiple codes can be formed into trees in the same way as a Chart of Accounts. The Tax Codes structure is used to define your VAT return; it can be numeric and alphanumeric. You can define tax codes from the menu Accounting ‣ Configuration ‣ Taxes ‣ Tax Codes

Taxes, the basic tax object that contains the rules for calculating tax on the transaction it is attached to, linked to the General Accounts and to the Tax Codes. A tax can contain multiple child taxes and base its calculation on those taxes rather than on the base transaction, providing considerable flexibility.

The General Accounts, which record the taxes owing and paid.

More at https://doc.openerp.com/7.0/book/3/3_9/tax_structure/