Generally speaking to enter any foreign country you'll need some kind of entry permission. You may be from a country that is part of the UAE's visa-waiver program, in which case you'll be allowed to enter once you've satisfied a few criteria (usually that you have an onward ticket and some money). Check with the UAE embassy in your country. about 6 years ago
As Phil says, depending on your passport you can either get a visa on entry or you will have to apply before arriving.
Emirates Airlines has this helpful information: http://www.emirates.com/english/plan_book/essential_information/visa_passport_information/uae_visas.aspx about 6 years ago
Surely you do require a visa to visit Dubai, unless its a transit in Duba for a couple of hours in which you dont leave the airport. Even during a transit if you decide to leave the terminal to get a hotel somewhere in dubai, you will require a visa. about 6 years ago