Circled C ⓒ vs. the copyright sign ©︎
Many people type a circled C when they mean copyright. The copyright sign ©︎ (U+00A9) is its own character with its own meaning. The circled letters ⓒ and Ⓒ belong to a set of enclosed letters (ⓐ to ⓩ, Ⓐ to Ⓩ) made for lists and labels. They look similar, but screen readers and search engines treat them as a letter in a circle, not as a copyright notice. For a real notice, use ©︎; the copyright symbol page explains how.
For decoration, though, circled and squared letters are great: 🅒 and 🅲 have a bold, sign-like look that suits initials, monograms and usernames.
Fancy Cs from math alphabets
Most "fancy font" letters come from Unicode's math alphabets, such as script 𝒞, bold script 𝓒, bold 𝐂 and bold gothic 𝕮. A few capital Cs are special because they were added to Unicode earlier, in the Letterlike Symbols block: ℂ (double-struck C) is the standard symbol for the set of complex numbers, and ℭ is the black-letter (gothic) C. Because they come from a different block, they can look slightly different from the other letters in the same style.
To turn a whole word into one of these styles, use the cursive text generator or browse the letter symbols page.
Symbols built on the letter C
Several everyday symbols are a C with something added. The cent symbol ¢ has a vertical or slanted stroke; on a Mac it is Option + 4, and on Windows Alt + 0162. The cedi sign ₵ is the currency of Ghana, and ₡ is the colón of Costa Rica. ℃ is a single "degrees Celsius" character, although writing °C with two characters is preferred. ℅ means "care of" on envelopes, Ⅽ is the Roman numeral 100, and the reversed Ↄ appears in old Roman numeral forms like CIↃ for 1,000.
Watch out for the Cyrillic С and the Greek lunate sigma ϲ: they look identical to a Latin C but are different letters, so searches and spell checkers will not match them.