Visualforce is Salesforce's first-generation custom UI framework, introduced in 2008. XML-style markup compiles to HTML, with Apex controllers driving server-side logic. Each interaction round-trips to the server.
Where it stands today: legacy but supported. Many existing orgs have Visualforce running. In Lightning Experience, VF pages render inside an iframe wrapper. New work should use LWC.
When you'd still touch Visualforce: maintaining existing pages, Visualforce Email Templates (still useful for templating), PDF generation via <apex:page renderAs="pdf"> (simplest server-side PDF rendering), specific platform features without LWC equivalents.
Migration: many orgs are systematically rewriting Visualforce as LWC. New developers should learn LWC; familiarity with Visualforce only for legacy.