Although the French word for "lawyer" is the same as the one for "avocado", their etymologies differ.
When it comes to the profession, "avocat" comes from the latin verb "advocare" (which gave the verb "to advocate" in English).
Ex: Je veux parler à mon avocat. => I want to talk to my lawyer.
The fruit "avocat", however, comes from a Spanich word, which derived from a native American word.
Ex : Cet avocat est mûr. => This avocado is ripe.
The two etymologies have very different roots, but they helped formed two nouns spelled and pronounced exactly the same way.
As to the difference between "avocat" (people) and "juriste": once you pass the bar exam in France, you become an "avocat". A "juriste" has studied law, he can then become a legal practitioner (my brother is a "juriste" in agricultural law), pass the bar, or work for the justice department (after a competitive exam). But, as far as I know, into US English, "avocat" translates to "lawyer" or "attorney".