People always want to know if a visa is needed to visit Palestine, and, if so, what one needs to do in order to get a visa. They also want to know for how long a visa is valid.
The Israeli government and its authorities are responsible for providing you with a visa either upon your arrival (the Palestinian Authority is not responsible as it does not control the state borders of Israel), or through its embassies/consulates prior to your visit.
Visas are routinely granted upon arrival at Ben Gurion Airport (Tel Aviv) if you are a citizen of an exempted country, or at the Allenby-King Hussein Bridge border crossing; Hamas controls the border in Gaza with Egypt. Tourist visas are valid for 90 days; most Americans and Europeans are issued three-month visas upon arrival and clearing customs.
If your home country does not have an exemption agreement with Israel, you will have to procure a visa from the Israeli embassy/consulate at home before you visit. Because many Arab countries do not accept Israeli stamps upon passports, Israel now issues their visa on a separate small card with bar code indicating the visa time validity. Please keep this visa card WITH your passport at all times.
At this website you may check to see whether or not your native country participates in a visa exemption agreement: http://mfa.gov.il/MFA/ConsularServices/Documents/VisaRequirements-Tourists.pdf
You can watch this short video for more information: