4chan is the oldest and most popular english speaking imageboard. 4chan has a finite number of boards and is managed by a closed list of mods and vols.
8chan/kun has historically held second place in size. It adds the features of torposting, vpn posting, and most importantly user created boards. 8chan picked up it's early momentum with the gamergate event. 8kun is just rebranded 8chan, if you didn't know.
On both sites the culture and posters generally stick to their own board of interest, but 4chan is culturally led by /pol/ and 8kun by qboomers. Our /pol/ equivalent /pnd/ is the edgier version of 4/pol/