A visa is required for Cape Verde. It can be purchased at arrival for € 25,- (make sure to bring cash). It can be purchased in advance as well, but usually it is more expensive and laborious. With some airlines a visa is included in the price. A visa allows for a maximum stay of 90 days and is valid for entry within 180 days after date of issue.
Note that at arrival in Cape Verde it is sometimes necessary to buy a visum first before you connect in line for customs (in São Vicente it is the small office on the left at arrivals).
You don't need a visa if you were born in Cape Verde, are married to someone who was born in Cape Verde, or if one of your parents was born in Cape Verde. You may need to prove this by providing a document (for example a marriage certificate). Note that if you booked a flight which inlcudes a visa you may be able to get a refund in case you don't need a visa.
Officially, the passport must be valid at least 6 months after departure.
The international embarkation/disembarkation card that had to be issued to the customs on arrival an departure has been abolished in 2013. It is still used once in a while if there is a problem with the computer system.