It's plot is given quite early, but is often overlooked. The plot it that Oga is supposed to raise Beelzebub into the new Demon King. Simple as that. This includes everything from Oga and Beelzebub growing stronger, to gaining minions and so forth. The anime and manga are quite easy going, but slowly and with no rush, it's moving further against the goal.
You may ask yourself what Oga feels about having his "son" to become a powerful and merciless Demon King, however thoughs regarding this hasn't been mentioned more then a couple of times with no further talk about it.
Personally, I prefer to read the manga due to the anime being cancelled. I've yet to get tired of the manga due to all the awesome and funny things occuring in it. I gave up on the anime once I knew it wouldn't continue.
To answer your question;
Beelzebub is a comedy with a (slim) story.