EU nationals do not need a visa or work permit. This is applicable for those who wish to stay less than 90 days. If you want to stay for more than three months, you must apply for a residence permit.
For non-EU citizens, short-stay visas are available for stays up to 90 days and long-stay visas for those who wish to stay for longer than 90 days. Contact your embassy for details of visa and work permit requirements.