The Piraeus (Πειραιάς, official transliteration Pireas) is the ancient port of Athens, and still functions as the chief exit point from the city by sea, for destinations among the Aegean Islands and elsewhere in the east Mediterranean. Domestic destinations include all of the Aegean islands except the Sporades and some smaller Cyclades and Dodecanese isles that require a connection. International destinations (apart from cruise ships) include Cyprus and the Middle East.
Piraeus occupies a huge territory. Check your gate number in advance, so you know where to go. It may take 15 minutes of driving or 30 minutes of walking to reach one end of the port from the other.