Total clicks counts every click on a link without differentiating the source address, the account or anything to identify the click performer.
Unique clicks, however, based on its literal meaning, counts only unique clicks. That is, clicks from the same performer, IP address, etc., is only counted once.
The following is an answer from Quora:
Alexander Czernay, head of web analytics at sinnerschrader // certified google conversion professional (GCP) // inventor of the continuous optimization process pattern
Written 20 Apr 2011
Basically, all web analytics software measures differently. Also the same tool may measure differently, if its tracking is implemented another way (i.e. start of page vs end of page). And they all measure inaccurately. But that is ok. You want to read trends from comparable data, when using web analytics. If you want hard transaction data, use your back office system.
Now, in more detail, on the metrics you asked for:
_ HIT: outdated, useless measurement parameter from logfile-analysis times; counts every single element downloaded from a webserver to display a given page (i.e. every image, script, stylesheet,…) don't use it
_ CLICK: no common definition; usually used in conjunction with media elements like banners or ads; counts clicks on these elements; often called click-throughs
_ VISIT: a given user enters the website, uses it for some time without pausing for more than (depending on tool vendor) 30 minutes. Often a visit is also ended after 12 hours or at midnight
_ VISITOR: any uniquely identified user; where uniquely identified usually means tagged with a persistent cookie, or – if that fails – identified by a combination of IP address and user agent
_ PAGEVIEW: a single page viewed
Many tool providers add specifica to these basics and they all implement their measurement differently. Adobe/Omniture, for example, adds Daily Unique Visitors (DUVs), Weekly Unique Visitors (WUVs), Monthly Unique Visitors (MUVs), Quarterly Unique Visitors (QUVs) and Yearly Unique Visitos (YUVs) to the basic visitor metric. These further filter your visitors to only count a user once in a given timeframe. These specifica might give you further insight and enhance your analysis. Just don't try to directly compare different tools' data even if it's labeled the same.
