Snoopy is a loyal, imaginative and good-natured beagle who is prone to imagining fantasy lives, including being an author, a college student known as "Joe Cool", an attorney and a British World War I flying ace. He is perhaps best known in this last persona, wearing an aviator's helmet and goggles and a scarf while carrying a swagger stick (like a stereotypical British Army officer of World War I and II).

Snoopy can be selfish, gluttonous and lazy at times, and occasionally mocks his owner, Charlie Brown. But on the whole, he shows great love, care, and loyalty for his owner (even though he cannot even remember his name and always refers to him as "The Round-Headed Kid"). In the 1990s comic strips, he is obsessed with cookies, particularly the chocolate-chip variety. This, and other instances in which he indulges in large chocolate-based meals and snacks, shows a resistance to theobromine unheard of in other dogs.

All of his fantasies have a similar formula. Snoopy pretends to be something, usually "world famous", and fails. His short "novels" are never published. His Sopwith Camel is consistently shot down by his imaginary rival enemy, the German flying ace the "Red Baron". Schulz said of Snoopy's character in a 1997 interview: "He has to retreat into his fanciful world in order to survive. Otherwise, he leads kind of a dull, miserable life. I don't envy dogs the lives they have to live."

Snoopy imagines himself to speak, but never actually does, other than nonverbal sounds and occasionally uttering "Woof". His very articulate thoughts are shown in thought balloons. In the animated Peanuts films and television specials, Snoopy's thoughts are not verbalized. His moods are instead conveyed through moans, yelps, growls, sobs, laughter, and monosyllabic utterances such as "bleah" or "hey" as well as through pantomime. His vocal effects were usually provided by Bill Melendez, who first played the role during Snoopy's appearances on The Tennessee Ernie Ford Show.

The only exceptions are in the animated adaptions of the musicals You're a Good Man, Charlie Brown and Snoopy!!! The Musical in which Snoopy's thoughts are verbalized by Robert Towers and Cameron Clarke, respectively. (His dialogue, however, is not "heard" by the other characters except Woodstock the bird and other non-human characters, however he does remember Charlie Brown's name.)

Snoopy's doghouse defies physics and is shown to be bigger on the inside than the outside.