Question:What is Angular Prefixes $ and $$? 

To prevent accidental name collisions with your code, Angular prefixes names of public objects with $ and names of private objects with $$. So, do not use the $ or $$ prefix in your code.

Question:What are Expressions in AngularJS? 

AngularJS expressions are much like JavaScript expressions, placed inside HTML templates by using double braces such as: {{expression}}. AngularJS evaluates expressions and then dynamically adds the result to a web page. Like JavaScript expressions, they can contain literals, operators, and variables.

There are some valid AngularJS expressions:
- {{ 1 + 2 }}
- {{ x + y }}
- {{ x == y }}
- {{ x = 2 }}
- {{ user.Id }}

Question:How AngularJS expressions are different from the JavaScript expressions? 

AngularJS expressions are much like JavaScript expressions but they are different from JavaScript expressions in the following ways:
1. Angular expressions can be added inside the HTML templates.
2. Angular expressions doesn't support control flow statements (conditionals, loops, or exceptions).
3. Angular expressions support filters to format data before displaying it.