Aragorn: King of Gondor?

[Meela]

I was flicking through a book earlier, and I came across a family tree. It stated that Elendil had two sons, only these were written as:

---Kings of Gondor.

---Kings of Arnor and Arthedain---Chieftains---Aragorn II

this confused me, as it appears that Aragorn is not the heir to the throne of Gondor, but we all know he is.

I read elsewhere that Anarion, the younger of the sons, was King of Arnor, and I know Isildur was king of Gondor.

But I have also just read that Anarion was in fact joint ruler of Gondor with Isildur, and Isildur is mentioned as the one with contact with Arnor, as he went there with his father to rally the Dunedain.

Now I am in a state of utter confusion over Aragorn's heritage and rulership. Could someone please set me straight? Perhaps it is just the book that stated it incorrectly, but it would be great to know for sure. Thanks.

Also, while I am on the subject of rulership, I heard/read that the Stewards were descendants of Elendil. Is this true? And if so, does that give them right to the throne, like Aragorn? Or is it a distant tie only?