The unit of Pakistani money is the rupee (Rs), divided into 100 paisa. Paper notes come in denominations of Rs 5000, Rs 1000, Rs 500, Rs 100, Rs 50, Rs 20 and Rs 10, and there are Rs 5, Rs 2 and Rs 1 coins.
Very worn or tattered notes or those with pen scribbles on them may sometimes be refused so it's best not to accept them in the first place. The Rs 5000, Rs 1000 and Rs 500 notes can be a headache (few people seem to have change handy), so ask for smaller notes when you buy your rupees.
Automatic teller machines (ATMs; most 24 hours) can be found in almost all the major cities, such as Islamabad, Lahore and Karachi. Major banks will accept Cirrus, Maestro, MasterCard and Visa (but not always all cards). However don't rely on ATMs as your sole source of cash, especially if you plan to travel beyond the big cities. Check with your bank before departing to confirm that your card can access international banking networks.
It's unwise to carry wads of money in your wallet, or to carry your wallet in your back pocket. Similarly, you're more prone to being robbed if you carry valuables in a shoulder bag, which can easily be snatched. Keep a small cash stash for the day in a handy but concealed place (eg in an inner pocket) and the bulk of your resources more deeply hidden.