The local currency in Cape Verde is the Escudo (CVE). It has a fixed rate against the euro: 1 euro is about 110 escudos (110.265 to be precise). A 1000 escudos bill for example is about 9 euro.
It is often possible to pay in euros as well, but usually a rate of 1:100 is used (1 euro is about 100 escudos).
Creditcards are not widely accepted. On tourist spots it is more common, but still in most cases the only accepted card is Visa. In some restaurants the Visa card is accepted, but often the extra costs are passed to the customer (usually 3%).
ATMs are fairly present (airports and the larger towns). Most European debit cards can be used to withdraw money. A Visa credit card can be used as well. The maximum amount that can be withdrawn is usually 20,000 escudos (around € 180,-).
Cape Verde does not have a tipping culture and a tip is not required. If you want to give something as a token of appreciation a small amount of 50 or 100 escudos is usually sufficient. On tourist spots you can think of 5% as a guideline.