Just one thing before i start explaining breifly the difference between the two terms , there is nothing out there called offline SEO , it just doesnt make anysense however , there is the offline marketing if you would like to know more about it .
Now back to our subject , onpage or on page SEO refers to the content ( text , image alts , titles etc…) on your website , the process of making your website visible and highly ranked within SERPS ( Search engine result pages ) , going onwards , its a big world so be careful when you use it ( research blackhat SEO ) .
offpage or off page SEO is related to pagerank , directories & mainly links ( good ones ) pointing at your website or a landing page , this process is usually called link building? .
now i dont want to get it more complicated , more posts about search engine marketing & search engine optimization are coming soon ( well , when i have time ) .