Paranoia, also known as paranoid delusions, are fixed, false beliefs- these thoughts and beliefs are not real, or an over-amplication of something that is true, but the extent of the belief is way beyond the truth. So paranoia is an extreme belief that one's life is endangered or threatened, and the fear that results is not based on reality.
Agoraphobia, on the other hand, is an extreme fear of going into public places, for fear of not getting help or fear of having a panic attack there. Agoraphobia is basically a fear of public places, and suffers often are homebound, hardly ever leaving their home.
So paranoia and agoraphobia do seem similar in their presentations, as both have extreme fear, and both are home-bound. However, paranoia is based on fixed, false beliefs, while agoraphobia is based on fear of having a panic attack or fear of not getting help while out in public. They may look the same, but they are different in how they develop. It is important to tell the difference, as they have different treatment plans, as one can be part of a psychotic disorder (paranoid delusions), while the other is an anxiety disorder (agoraphobia).