Kiddushin 2 - Ways to Acquire a Woman
A wife can be acquired in one of three ways and she acquires herself back in one of two ways.
She may be acquired through money, document of betrothal, or cohabitation. Two witnesses are needed to testify in case of money and document. In case of acquisition by cohabitation (which is not encouraged) the witnesses testify to the declared intention and to the seclusion.
She acquires herself back through a Get or through death of her husband.
Passive use of "she is acquired" as opposed to "he acquires" teaches that a woman can only be acquired willingly, and not against her will.