Clients and relays
Clients
Clients are the way that you access and interact with the Nostr protocol.
What's a client?
A client in Nostr is just the app that you use to access and interact with the protocol. It's like the Twitter iOS app or web app that you use to interact with Twitter's feed of tweets.
What is the point of having multiple clients?
Each client has its own peculiarities, each exposes different parts of the data and allows you to interact differently with it or create different kinds of content.
However everything is interoperable and the data in the underlying network always the same, so your profile will be the same no matter what client you use, your list of contacts and the data you published will be with you in every client.