Public key - known by anyone and everyone over internet, this is YOU, your real identity, your name
Private key - used to decrypt all data which is not public in nature, your DMs, your post signatures. It should be protected AT ALL TIMES. It is used to verify if it is YOU.
Not an app or website, but it is much like a set of rules aka PROTOCOL. Think of like it is an email service. What is EMAIL, not an app, but a protocol like IMAP, POP etc. sent over servers. By the way, emails are termed as NOTES here, hence the name: Notes and Other Stuff Transmitted by Relay (NOSTR). See more over here: https://nostr.com/
NOSTR protocol has now many apps built within its ecosystem, like Twitter like apps (microblogging) e.g. - Snort, Damus etc. It is like your identity (like email, phone number). So many websites take it as a sufficient and secure way to login and interact. Some apps to look at are: StackerNews, NostrChat etc. Whole new world out here: https://www.nostrapps.com/
What is key-pair ?
What is NOSTR
What are servers here ?
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