1) שומר חנם, or unpaid watchman - is one who watches an item without receiving payment for his watching. Shomrei chinam are liable only for damages that result from their own negligence (Pshi'a) but are exempted if the item is stolen or lost not due to his negligence (Geneiva VaAveida). The shomer chinam does not have permission to use the item he or she is entrusted with.
2) שומר שכר, or paid watchman, also known as a נושא שכר, one who receives payment, is one who watches an item in exchange for compensation. He is liable if the item is stolen or lost (Geneiva VaAveida). Armed robbery is exempted from this responsibility, as is any Ones, since it is considered beyond the custodian's reasonable control.
3) A שוכר is a renter. The liabilities of a socher are the same as those of a shomer sachar.
4) A שואל is a borrower and is responsible for anything that happens except if it happens in the course of what he borrowed it for (Maisa Machmas Milacha) or if the owner was in his employ at the time of the borrowing (Ba'alav Imo).