One of them is the most affectionate ball of fluff, and the other one doesn't care about you in the slightest.
The life of one of them revolves around you, and he lives in shadow when you're not there. The other one, however, doesn't live with you: you live with him. It's his territory and he'll do with it what he wants.
If you don't know what we're talking about, these 8 images should clear things up!
Source: @Protectivity
Source: @Twitter
Source: @Cutestuff.co
Source: @Videobash
Source: @HemmingfordDogBlog
Source: @TheDodo
Source: @Imgur
Source: @Buzzfeed
Source: @Pedigree
Source: @Forgifs
Source: @Tumblr
Source: @Pbh2
Source: @Buzzfeed
Source: @Buzzfeed
Source: @Cheezburger
Source:@Tumblr4gif