PageRank is a link analysis algorithm used by the Google and is a number between 0 and 10.
These are not domain pageranks but inner page ranks.
Alexa rank websites based solely on traffic to the website. Alexa rank websites in descending order towards 0, where the most highly visited website on the Internet is given a rank of 0.
A backlink is link that you received from other websites, the more you got the better it is for your rank.
It’s possible to get your ranking from Google, but that’s harder than you would think. Google generates a checksum from the URL, with this checksum in combination with the URL makes it possible to get your PageRank.
When you manage multiple webpages you can check them at once.
The Google Position Checker will automatically query Google's search engines.
It checks if your URL appears in the first 100 results for your chosen keywords.
Google runs many servers, and updates your PageRank at different times. Check your PageRank on over 15 Google servers!
There's a igoogle tool available, Embed this gadget on your own website.