If making a pledge to Allah Almighty is done to encourage yourself to do something or to stop doing something, then it comes under the same ruling as an oath. If a person says: I pledge to Allah Almighty that I shall do Hajj this year, then this is a vow, a pledge and an oath. If he says: I shall not speak to Zayd, it is an oath and a pledge, but it is not a vow. If an oath includes the meaning of a vow, which is a commitment to Allah to do an act that will bring him closer to Him, then it must be fulfilled. Secondly If you made the pledge mentioned to Allah Almighty, then you must offer expiation for breaking an oath (Kaffaarat Yameen), because this was a vow to do something permissible, which is giving up any permissible habit. But there is no obligation to fulfil it; rather the person has the choice between fulfilling it or offering expiation for breaking an oath.