Typically a short term visa can be obtained at the border. Citizens of some nationalities need to obtain a Social or Business Visa before entering. A list of these countries can be found at http://www.ica.gov.sg
The visitor needs to satisfy the following requirements:
- A passport with at least 6 months of validity remaining
- Enough money to last the intended duration of stay
- Confirmed onward/return ticket
- Completion of the disembarkation/embarkation card
- A Yellow Fever Vaccination Certificate, depending on which country you are coming from
- A valid Singapore visa (if required as per the above)
Once the short term visa is approved at the border, it will contain an approved duration of stay. It is important that the duration of stay does not exceed the approved time period, as it is a punishable offence in Singapore to do this. It is quite easy to get an extension, and you must apply for it before your approved time period is over.