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Marketing campaigns (traffic driving initiatives that brings customers to your site) are one of the most important segments that you may want to see in your reports.

For each user that comes to your site GA automatically captures a number of attributes or dimensions about where the user came from, two such dimensions are Source and Medium.

Source is the name of the website that referred the user to your site, ex: if a user come to your site from google search, the Source will be 'google'

Medium is the mechanism or how the user came to your site, ex: if a user come to your site from an upaid search result then Medium will be 'organic'
Google detects 3 mediums by default namely 'organic', 'referral' and 'none'
where 'referral' is if a user comes from another website, and 'none' if the user come directly to your site.

To track campaigns in GA, we use a process called Link tagging.
Link tagging is adding extra information to the links user clicks on to get to your site. This extra information you provide is stored in campaign tags and overrides the default categorization that would normally be assigned to the incoming traffic.

There are 5 campaign tags
1.source
2. medium
3. campaign
4. term (optional)
5. content (optional)

URL Builder can be used to build campaign tag urls.

Adwords autotagging automatically imports the above 5 tags, so additional work needed.

Channels in GA allows you to organize rule based groups of traffic based on your own campaigns, sources, keywords or mediums.
ex: you can create one channel for your email, search and display marketing team's join strategy and call it Summer promo that groups these coordinated activities together for analysis in your reports.
GA automatically detects several predefined channels ex: Direct, Email, Social