Reach

Reach is the total number of unique users your post is shown to. Reach is an important metric for marketeers to understand exactly how far a post has spread in their target audience.
 

Impressions

The total number of times your content was displayed on the screen of a social media user, is referred to as impressions. As opposed to reach, impressions don’t revolve around unique users. So if one person sees your post five times, this counts as five impressions.
 

Organic Reach

Organic reach refers to the number of social media users that sees your content without having to do paid advertising.
 

Paid Reach

Paid reach refers to the number of social media users that sees your content as a result of paid advertising.