Easily the most recognizable aspect of Daria Morgendorffer's persona, her glasses are heavy black eyeglasses with large, round lenses that easily cover a notable portion of her face.
In the show
Daria's prescription has never been revealed in canon. However, as a number of episodes have shown (most notably "Through a Lens Darkly"), Daria has quite poor eyesight and needs her glasses to function normally, as she kept bumping into classmates when pretending to wear her contacts and had to be guided at times by Jane Lane. Even with her glasses on she has visual problems, once nearly running a dog over while driving because she had poor peripheral vision ("Through a Lens Darkly").
A "future-ego" shot at the end of "Is It College Yet?" shows her as an adult wearing glasses, though more stylish than she wears now, as does her imagined future self in "Write Where It Hurts". Her other imagined future in "Lane Miserables" gives her larger, square glasses in what she felt was the realistic future with Trent and more fashionable smaller glasses in the romantic fantasy future.
"Daria Morgendorffer's Greatest Retail Regret", created and set after "College", shows an adult Daria wearing the same glasses that she had in the series. "Catching Up With The Daria Gang", created even later, showed a mid-30s Daria wearing chunk square glasses, not that far off from her "Lane Miserables" future. Evidently, adult Daria isn't too interested in fashion (a shock I know).
"Through a Lens Darkly" presented the most detailed discussion of Daria's glasses and what they mean to her. Jane Lane said it best in a discussion with her amiga:
- Daria: Now my mother's bugging me about contacts again.
- Jane: Boy, she just doesn't get it, does she?
- Daria: Nope, she really doesn't. [pause] Get what?
- Jane: You! The whole Daria Morgendorffer persona. You don't care what people think about your looks.
- Daria: Of course not.
- Jane: The glasses are you. They're symbolic of the whole Daria thing. [imitates Daria] "I wear glasses and I'm not going to apologize for it."
Amy Barksdale, often taken to be similar to an older Daria, had no problem with replacing her glasses with contacts, but Daria found contacts too troublesome (and painful) to use. Amy has also kept her glasses and sometimes dons them for visual effect (in "I Don't" to bond with Daria).
In "Through a Lens", Helen does make it clear she thinks Daria will get on better socially without glasses.
Understandably, many fans of Daria wear glasses. Fans have sometimes commented about their own glasses (or lack of same). Kara Wild (she who gave Quinn Morgendorffer eyeglasses in her Driven Wild Universe cycle) began one such thread on PPMB, which is worth reading for the various reactions people have to glasses. Medea42 initiated another such thread on PPMB, and Brandon League began a like thread more recently, both also worth a look.
A recurring fandom term is "Manstopper Glasses", first coined by Brother Grimace. Fans who use the term argue that Daria is deliberately dressing to put up an unapproachable, 'unattractive' front - on assumption she's conventionally attractive ala Quinn and is trying to hide it - and the unfashionably severe glasses are an attempt to ward men off.
An official Daria costume came out in October 2011, complete with a replica of Daria's glasses.