What is a Telegram Client?
RAIYAN (@raiyanmods)A Telegram Client is built using Telegram's source code, which Telegram officially releases weeks (sometimes months) after each update.
Third-party developers take that Codebase & Customize it — Adding new features, sometimes Unlocking local Premium perks, and more. It's done both at Windows & Android platform & maybe others as well.
Popular Examples: Plus Messenger, NekoX (aka Nekogram), AyuGram, Ninjagram, Graph Messenger (aka Telegraph), Unigram(MS Windows), 64Gram(MS Windows).