Here is the full google ad services pagead aclk URL we would have used to click the ad:
http://www.google.com/aclk?sa=l&ai=C…rt=v&rate=1.99
*Asterisks replace alpha numeric characters to protect Google its advertiser within the google ad services pagead aclk url.
So here they have the breakdown of google ad services pagead aclk
1) http://www.google.com/aclk?
Stage one here is just a page that actually has nothing on it, behind the scenes however a lot is going on:
The user clicks an ad
All of the information in those long codes all relate back to a tracking mechanism
Your IP, User agent, Screen Resolution, Operating System plus ANY cookie Google has placed on you machine is read
Google decides algorithmically if you are a genuine person
Google predicts if you are likely to convert for its advertiser
Google runs your information through highly complex artificial intelligence systems to detect click fraud
Then after it has stored all of your information, placed another couple of tracking cookies you are 301 redirected to the advertisers destination URL
2) sa=l&ai=CiYM8O******* TRUNCATED ******mG7ISbLjDg&ved=0**EQ0Qw
I am affraid not even the smartest of Google employees could tell you what this part of the google ad services pagead aclk actually means, its just a very long unique charater string that relates back to a Google database somewhere. In this database I am sure it is linked to a million other things and I am 100% positive that my mind would explode with the amount of information they store.
3) &num=6
This part of the google ad services pagead aclk is pretty straight forward, the advertiser is position 6 in the sponsored results. Used mainly to calculate your average position and entice you to bid higher 
4) &adurl=https://www.********.co.uk/la/1/?keyword=mortgage&gcrid=74*******65&rt=v&rate=1.99
This is the destination URL or the landing page the advertiser has entered into the adwords systems displayed within the google ad services pagead aclk, its quite an interesting portion of the google ad services pagead aclk URL so lets explore these aspects very carefully
https – This tells us that you are being redirected to a secure / encrypted website using a secure protocol. Don’t take this as it is though you should always check the SSL certificate and ensure you get a padlock in your browser
www.*******.co.uk – This is the URL of the advertiser
/la/1/ – This shows me that this advertiser knows what they are doing. I am guessing here but /la stands for landing page and /1 means version 1. I would fully expect this advertiser to be trailing multiple landing pages and tracking the conversions of each one. Adwords is very helpful in this area, if you use its conversion tracking system it will actually display your converting ads more regularly therfore sending people to the super converting landing page (Isn’t Google Wonderful).
keyword=mortgage – This keyword (mortgage) has been dynamically inserted into the URL, again this allows the advertiser to track conversions based upon the keyword, thus negative matching or just removing non converting keywords
gcrid – This is section of the google ad services pagead aclk likely to map back to a database inside the Advertisers database. It is not something used or provided by Google
&rt=v&rate=1.99 – Mortgage related terms and thus provided by the advertiser
So in conclusion to What is google ad services pagead aclk?
Google ad services pagead aclk is all about tracking, tracking and more tracking. The more Google tracks the more its combats click fraud, provides features for its advertisers and allows smart people to analyse the data to come up with more ingenious ways to enhance its offering. We are slowly starting to see at random similar style metrics / url options inside the SERPS. This has a great deal to do with webmaster tools, this arm of the Google machine is providing all of the search metrics for you to basically compete with other SERP results. Just keep in mind that in the big picture the google ad services pagead aclk is really not something that you have to worry about.
