Greeks tend to eat lunch at 1.00pm at the earliest, and dinner at 9.00pm at the earliest, and much later, especially in the hot summer months when they love to sit out in the cool wee hours (they manage this trick by taking long naps during the hottest hours of the afternoon). But most tavernas, especially where there are likely to be tourists around, open from about 6:00pm (some never close) so if you want to eat earlier you certainly won't starve. But you may only have other foreigners as company.
Unless you go to a smart restaurant, eating out tends to be extremely informal. You sit down, the server whips out a paper tablecloth and fastens it over the table (on islands these often feature useful maps to study; the paper tablecloth is there for hygiene and to save money and energy washing tablecloths). Someone will plop a basket with napkins and cutlery on the table for you to pass around, usually with some bread to nibble. They may also plop down a bottle of mineral water (which you will be charged for; if you don't want it and prefer tap water, ask for νερό από τη βρύση (nero ap ti vrissi).
In Greece people tend to order dishes 'for the table' and not as individuals, although obviously if you want all that lovely golden fried kalamari just for yourself that's okay, too (if you aren't eating with any Greek friends!).
Besides studying the menu, always ask the server for suggestions; often the best things aren't on the menu (food is very seasonal, and perhaps a special fish was caught that morning). As it's expected in tavernas that everyone around the table will share several dishes (especially the appetizers), you'll get an empty plate so serve yourself from the platters. Eating with your fingers is okay too (just ask for more napkins). Do warn the staff if you have any special food requirements or allergies.
Although increasingly Greek restaurants and tavernas, especially in tourist areas, accept credit cards, they don't like them; the owners think why should a multinational corporation profit from their hard work and already tiny margins?
Μπαρ bar
Μπουγατσάδικο (bougatsadiko) Specialists in bougatsa
Εστιατόριο (estiatório): restaurant
Φαστ φουντ: fast food
Καφενείο (kafeneio): old style coffeehouse
Καφετέρια (kafeteria): café
Μεζεδοπωλεία (mezedopoleía): bar specializing in mezedes
Ουζερί (ouzeri): bar specializing in ouzo and mezedes
Πατσατζιδίκο (patsatzidiko): tripe restaurant
Πιτσαρία (pitsaria): pizzeria
Χασαποταβέρνα (chasapotaverna): meat taverna, sometimes linked directly to a butchershop.
Ψαροταβέρνα (psarotaverna): taverna specializing in seafood
Ψησταριά (psistaria): grill house, barbecue
Σουβλατζίδικο (souvlatzidiko): souvlaki stand
Ταβέρνα (tavérna): informal restaurant
Τσιπουράδικo (tsipouradiko): bar specializing in tsipouro and mezedes
Εντράδα (entrada): entrées
Επιδόρπια (epidorpia): desserts
Φρούτα (frouta): fruit
Γλυκά (glyka): sweets
Κατεψυγμένος (katepsymenos): frozen. Menus should state if a fish is frozen (abbreviated 'κατ')
Κατάλογος (katalogos): or μενού menu
Κυμάδες (kymades): minced meat dishes
Κύριο πιάτο (kyrio piato): main course
Κουβέρ (couver): cover charge. Places often charge a small 'bread and cover' charge
Λαδερά (ladera): 'cooked in oil'
Λίστα κρασιών (list krasion): wine list
Μαγειρευτά (mageirefta): cooked foods (ie baked or oven dishes such as moussaka, the stalwarts of the old fashioned steam table)
Ορεκτικά (orektika): appetizers
Ποτό (poto): drinks
Ψαριά (psaria): fish. Small fish such as marida or gavros will be priced by the portion (μερίδα, merida), while other fish are priced by the kilo. You may be invited back to the restaurant fridge to choose your fish, or the server may bring the fish out for you to confirm or select.
Ψωμί (psomi): bread
Σαλάτες (salates): salads
Σούπες (soupes): soups
Της ώρας (tis oras): cooked to order (usually on the grill)
Τυριά (tyria): cheeses
Ζυμαρικά (zymarika): pasta and rice dishes
(also see Cooking terms and Reading the label)
bill: λογαριασμός (logariasmos)
bon appétit!: Καλή όρεξη! (Kali orexi)
bottle: μπουκάλι (boukali)
breakfast: πρωινό (proino)
calories: θερμίδες (thermides)
chair: καρέκλα (karekla)
chef: αρχιμάγειρας (archimageiras)
children's menu: παιδικό μενού (paidiko menu)
cold: κρύο (krio)
delicious: νόστιμο (nostimo)
dinner or supper: δείπνο (deipno)
fork: πιρούνι (pirouni)
glass: ποτήρι (potiri)
hot: ζεστό (zesto)
knife: μαχαίρι (machairi)
lunch: μεσημεριανό (mesimeriano)
medium (doneness) μεσαίο (mesaio)
napkin: χαρτοπετσέτα
non-smoking section: τμήμα μη καπνιζόντων (tmima mi kapnizondon). This usually means a table outside and upwind from all the smokers but the new government is trying to enforce EU rules
order: παραγγελία (parangelía)
pepper: πιπέρι (piperi)
pitcher: κανάτα (kanata)
plate: πιάτο (piato)
please: παρακαλώ (parakalo)
prices: τιμές (tim-es)
rare: σπάνιο (spanio)
receipt: απόδειξη (apodeixi)
salt: αλάτι (alati)
spoon: κουτάλι (koutali)
table: τραπέζι (trapezi); outside table εξωτερικό τραπέζι (exoteriko trapezi)
take away/out: στο πακέτο (sto paketo)
thank you: ευχαριστώ (efcharisto)
tip: φιλοδώρημα (filodorima). Sometimes menus will have two rows of prices, one with, one without the service charge, which is nearly always included although that rarely means your server receives that. If you're pleased with the service, leave a tip.
toilets: τουαλέτες (toualetes)
To your health! στην υγεία σου (stin eeyeia sou--singular), για την υγεία σας (yia tin eeyeia sas--plural)
water: νερό nero
waiter: σερβιτόρος (servitoros)
waitress: σερβιτόρα (servitora)
well done (meat) καλοψημένος (kalopsimenos)
Image by Andy Montgomery