It's as easy as elephants and bugs:
In the Infinite Seriesian Jungle, there are only two kinds of animals, elephants (divergent series) and bugs (convergent series). There are two basic ways to recognize elephants, i.e., to show that series diverge. Other tests for divergence are really special cases of the comparison test.
If the parts of the beast aren't small, it's an elephant. (If an does not tend to 0, the series does not converge.)
(comparison test) If a beast in the jungle is bigger than an elephant, it's another elephant!
Just as there are two kinds of elephants, there are two ways a series can fail to converge. The partial sums may tend to infinity, but another kind of divergence ("by oscillation") happens if they simply wander around finitely without ever committing themselves
Nice one Devin...pulling it from Someone else's contest
Mathematical Series
The Contest Center
59 DeGarmo Hills Road
Wappingers Falls, NY 12590
Even thought it's been (gulp) 20 years, I can still remember my t-shirt that said 'Math 318's a Bitch'
At A&M, that would be Differential Equations....and yes, it's a Bitch.