Hi Anthony,
Sorry for the delay.
Always => This will show the popup on every pageview meaning if a visitor is browsing your blog they will see the popup on every page they visit.
Once per Browser Session =>This will show the popup only once per session, so your visitor will see the popup on the 1st page view and will not see the popup any more while they browse your blog. If they close their browser and say come back tomorrow then they will see the popup again on the 1st page view.
Session cookies – these are temporary cookie files, which are erased when you close your browser.
For more information on session cookies:
http://www.allaboutcookies.org/cookies/cookies-the-same.html
Hope that helps
