Liddell and Scott's Greek-English Lexicon is printed in three editions: abriged, intermediate, and complete--otherwise known as the 'little Liddell', 'middle Liddell', and the 'Great Scott'. The first two of these cognomina are orthophemistically descriptive (the i in Liddell is short; the name's trochaic), and the last is a Classics student's first impression of this great book.
Don't waste your time with any other Greek-English dictionary. The abriged version is fine for most students' needs. The intermediate offers more dialect varients and etymology, but by the time Greek word origins and variations are needed, the Classicist should acquire the full Lexicon. Liddell and Scott was compiled in the 19th century, and often exhibits its Victorian tendencies with frustrating obliqueness. Some words then considered too naughty for rendering into English are glossed in Latin ('What's a membrum viri?'). The ninth edition of the book, published in 1996, supplements some definitions but for better or worse leaves intact most of the roundabout 19th C English (quoth today's budding Hellenist: "Greek, Latin, German, French, and Italian aren't enough; I have to learn English, too?).
Editor Henry Liddell was the father of Alice Liddell, a name familiar to fans of Lewis Carroll as the young model for Alice in Wonderland.