As with other countries, an International Passport or valid travel documents recognized by the Malaysian Government and internationally, is required for any foreign nationals entering Malaysia. The passport must have enough pages for the embarkation stamp upon arrival and be valid for at least 6 months at the date of entry.
A visa is not necessary for travelers coming from certain countries. For those who require visa, three types of visas can be obtained, whichever is applicable: single entry, multiple entry or transit visa. Visas have to be applied for in advance but the final decision for allowance of entry is at the discretion of the Immigration Officer at the entry point.
Application for Visa: U.S. citizens
A visa is not required for U.S. citizens visiting Malaysia for social, business or academic purposes (except for employment) for a stay of up to three months. A valid U.S. Passport is required. Application for visas should be made at the nearest Malaysian Embassy or Consulate General's office.
Application for Visa: United Kingdom citizens
A visa is not required for citizens of the United Kingdom as they are allowed to stay in Malaysia for 90 days (3 months) without the need for a Visa. Should they exceed their stay then they need to apply for a valid Visa up front.
Application for Visa: Rest of the world
Malaysia has different visa rules for different countries. In this document you can read up on the requirements related to your country. Most western countries (i.e. European countries, South-American and North-American countries) fall under the 90 days ruling.