Why are some amino acids classified as essential amino acids and some are classified as non-essential amino acids? How many of each are there?

Why are some amino acids classified as essential amino acids and some are classified as non-essential amino acids? How many of each are there?
the difference between an incomplete and complete protein. examples of each.
What are complementary foods(proteins)? Provide some examples and specify what amino acids are paired in your example.
How would a vegetarian ensure they are meeting all their protein needs?
Do professional athletes need to adjust their protein intake? Why/why not? Should there AMDR change for protein or just their total intake?
what is "Thermic of Food" and where proteins fall on this scale.