Different click types: Unique, not unique, spiders…

What are the differences between the click types?
It is the question we just received in our support email from Jenetra.
In CM there are four types of clicks:
Uniques:
“Made from a real users” – these clicks represents the number of visitors who clicked on a tracking link or visited a webpage. We check that the IP from where the click came from is unique within last 30 minutes (this default time for the web-marketing industry).
Spiders:
“Not human clicks” – these are coming from robots or web-spiders (such as search engines crawlers, rss feed generators, validators, etc…).
Conversions:“Goal !!!” – is when that click lead to the conversion. Someone clicked that link or visited that page and then purchased or subscribed.
Non uniques:
“Multiple clicks” – All other clicks. Practically are all the clicks made from the same person after the first click.