Aragorn is the Tolkien character known as Strider before becoming king.
When the hobbits meet him at the Prancing Pony in Bree, Aragorn presents himself as Strider, a wandering Ranger of the North. His identity and royal heritage are gradually revealed as the story develops.
Aragorn is the heir of Isildur and belongs to the line that once ruled Gondor and Arnor. He helps protect the Ring-bearer, leads armies against Sauron, and ultimately becomes King Elessar after the War of the Ring.
Strider is therefore a name or byname used during Aragorn’s years as a Ranger, not a separate character. He is also sometimes confused with other human leaders, especially Boromir and Faramir, who are members of the ruling family of Gondor but are not heirs of Isildur.