It is calculated by combining the Goods and services tax (GST) and Provincial sales tax (PST). It is collected by CRA (Canada Reviewing Agency). The HST rates are different for different provinces because of the different PST for different provinces.
There was a lot of protest against HST as it was supposedly increasing the number of services on which tax would be charges under HST but were exempted under PST. Proponents believed that HST will simplify the current tax system; Opponents believed that it will simplify book keeping but will increase the tax amount.