Lolcat is a compound word made from the acronym "LOL" and "cat". Lolcat images comprise a photo of a cat with a large caption characteristically superimposed onto the image in a heavy, sans-serif font such as Impact or Arial Black. Such images and memes following the format are often digitally edited for comedic effect.

Captions act as a speech balloon encompassing a comment from the cat, or as a description of the depicted scene. The caption is intentionally written with deviations from standard English spelling and grammar, featuring "strangely-conjugated verbs, but a tendency to converge to a new set of rules in spelling and grammar".

The text parodies the grammar-poor patois stereotypically attributed to Internet slang. Frequently, lolcat captions take the form of phrasal templates. Some phrases have a known source, usually a well-known Internet meme, such as All your base are belong to us or Do not want, while others don't. The language of lolcats has also been likened to baby talk, however it draws on a variety of linguistic resources, not just the imitation of baby talk.

Common themes include jokes of the form "Im in ur [noun], [verb]-ing ur [related noun]." Many lolcat images capture cats performing characteristically human actions or appearing to use modern technology, such as computers.

There are several well-known Lolcat images and single-word captions that have spawned many variations and imitations, including "Ceiling Cat" (see below). Others include Fail (a cat with a slice of processed cheese on its face) and "I Can Has Cheezburger" (a portrait of a blue British Shorthair). Another popular format is "[Adjective] cat is [adjective/noun]."

"Ceiling Cat" is a character spawned by the meme. The original image was an image macro with a picture of a cat looking out of a hole in a ceiling, captioned "Ceiling Cat is watching you masturbate." There followed numerous examples with the format "Ceiling Cat is watching you [verb ending in / rhyming with -ate]" with Ceiling Cat superimposed in the upper left hand corner of an image macro depicting the appropriate action. The underlying theme is that the cat is looking down on one, almost in a form of judgment.

"Ceiling Cat" and the corresponding "Basement Cat" (a black cat who lives in the basement) are said to represent good and evil in the lolcat universe, and in some cases God and Satan, as in the LOLCat Bible Translation Project.

Ceiling Cat by Eva & Franco Mattes, a 2016 taxidermy cat installation mimicking the meme, is in the collection of the San Francisco Museum of Modern Art.

Variations on the lolcat concept include captioning photos of other animals in a similar style (e.g. loldogs for dogs, etc.).

The syntax of lolcat captions was used as the basis for LOLCODE, an esoteric programming language with interpreters and compilers available in .NET Framework, Perl, etc.

In the Java edition of the video game Minecraft, there is an option to change language settings to LOLCAT.

Another example is Polandball, which uses broken English, reminiscent of the lolcat meme.

lolcat, a variant of the Unix utility cat, outputs text in rainbow colours.