Zakat is both charity and tax. Zakat is considered charity - when paid voluntarily, without specifying an amount. But Zakat is also a tax - if a specific amount is a requested by the State. Zakat was initially a voluntary contribution, that eventually became an obligatory duty.
Salat / Salah is the Second Pillar of Islam and is therefore a cornerstone of a Muslims life. Salat / Salah is a set of five daily rituals and is mandatory upon all Muslims.
The Shahadah is the first pillar of Islam and is ones declaration, intention and proclamation to join or be a part of the Islamic faith. The Shahadah serves both a religious and political purpose.