The local currency is the Tanzanian shilling, but US dollars are accepted everywhere tourists go, and safari prices are quoted in dollars. Bring cash notes printed in 2009 or later — older notes are refused outright by banks and most businesses because of a long-standing counterfeit problem.
Bring a mix of denominations. Small notes (1, 5, 10, 20) are what you need for tips, drinks and curio stalls; larger notes get a better exchange rate if you change money. Cards work at upmarket lodges and in Arusha, Stone Town and the airports, but not in the parks, and ATMs in Arusha and Moshi dispense shillings from Visa and Mastercard.
Tipping is customary and genuinely matters to the people who look after you. The usual guidance is USD 20 to 30 per day for your safari guide from the group as a whole, USD 10 to 15 per day for a camp cook or crew, and USD 5 to 10 per day per guest into the general staff box at each lodge. On Kilimanjaro the crew tipping is substantially higher and briefed separately. These are norms, not obligations.