The four writing cases
UPPERCASE and lowercase simply change every letter; they are the fix for a sentence typed with caps lock on, and for headlines pasted out of a design file. Sentence case lowercases everything, then capitalizes the first letter of the text and of each sentence that follows a period, question mark or exclamation point, plus the pronoun I. It is the house style for body copy, bullet points and most email subject lines, and it is what the AP Stylebook recommends for headlines at many US newsrooms.
Title Case capitalizes each word but leaves short function words lowercase - articles (a, an, the), coordinating conjunctions (and, but, or, nor) and short prepositions (at, by, for, in, of, on, to, up, via) - unless they are the first or last word. That matches AP and Chicago style closely enough for blog titles, slide headings and book chapters. Proper nouns and acronyms are the one thing no automatic converter can get right: it will rewrite NASA as Nasa and iPhone as Iphone, so always read the result before shipping a headline.
The five programming cases
The lower five rows split your text into words first. The splitter breaks on spaces, punctuation and underscores, and also on internal capital letters, so parseHTTPResponse comes apart correctly as parse, HTTP, Response. It then rejoins the words with no separator (camelCase, PascalCase), an underscore (snake_case, CONSTANT_CASE) or a hyphen (kebab-case).
Where each is used: camelCase for JavaScript and Java variables and functions; PascalCase (also called UpperCamelCase) for class names, React components and C# members; snake_case for Python variables and functions and for SQL column names; CONSTANT_CASE (SCREAMING_SNAKE_CASE) for constants and environment variables such as DATABASE_URL; kebab-case for URL slugs, CSS class names and HTML attributes, because a hyphen is safe in a URL while an underscore can disappear under a link's underline. Converting the sample sentence gives quarterlySalesReportForTheNorthEastRegion, quarterly_sales_report_for_the_north_east_region and quarterly-sales-report-for-the-north-east-region.
Notes and limits
Accented and non-English letters are converted with the browser's own Unicode rules, so cafe and jose behave correctly. Numbers stay where they are and count as part of a word, which is what you want for identifiers like v2Api. Everything is computed locally in your browser - no upload, no logging - and each row has its own copy button so you can grab exactly the one you need.