A shomer (Hebrew: שומר, pl. שומרים, shomrim) in Halacha is a Jewish legal guardian, entrusted with the custody and care of another's object.
The laws of shomrim are derived from the Torah in Shemot (Exodus) 22:6-14. It is discussed in the Talmud in Seder Nezikin in tractates Bava Kamma and Bava Metzia.
There are four types of shomrim: shomer ḥinnam, shomer sakhar, shoel, and sokher, corresponding to an unpaid custodian, a paid custodian, a borrower, and a renter respectively. Each shomer has specific distinct laws that apply to it. In halacha there are two major factors that determine a guardian's liability: the ability to use the item, and the exchange of money. In general the ability to use the item (in the case of a renter and a borrower) or being paid to watch the item (in the case of the paid watchman) increases the guardian's liability, while paying for use of the item (i.e. a renter) decreases the guardian's liability.
Read more about Shomer: Types of Shomrim, Biblical Sources For Shomrim, Becoming A Shomer, The Shomrim's Oath