Desu is the Japanese equivalent to "be." It means "is," "are," "am," stuff like that. Deshita, the past tense, is for "was" and "were" and the like. 

People will add desu to the end of their sentence even when it doesn't belong because they think it's cute. When used like that it's a quirk.

I only know all this because I've taken Japanese since 6th grade. Just trying to spread knowledge~

And on the subject of not being here, same as Sakaki except it's more homework less illness.