There are 6 different tags you can add to your incoming URLs, to categorize your incoming leads as coming from Campaigns, see your UTM tags in Salesforce, and to get better structured data in general.
Utm_source, utm_medium, and utm_campaign should be used in ALL links that you can control, to give you the best data possible -- advertisements, email newsletters, blog links, partner referral sites, etc.
Utm_term should be used in Google Adwords and Bing advertising to dynamically insert the keyword. Utm_content is an optional fifth tag.
Example:
https://www.YOURSITE.com?utm_source=GoogleAds&utm_medium=PPC&utm_campaign=CampaignName&utm_term={Keyword}
Simply replace the highlighted portions with the values you wish for your web site domain and campaign names.
NOTE: utm_term={keyword} is required for Google / Bing ads, in order to capture the search term keywords into Salesforce.