In Memoriam is a very long poem. It is as long as a novel. However, you can read it in one sitting. The language and Tennyson's style is beautiful. The poem is dedicated to Tennyson's friend who died very young. The original title is "The Way of the Soul" and you can realize that the poem is very personal and dear to Tennyson.
This might be cliche but the part that I really like goes like this:
"'Tis better to have loved and lost
Than never to have loved at all."
This is without a doubt the most quoted lines from the poem. Even people who don't know anything about this poem and Tennyson like these lines and quote it. Brilliant poem in my opinion.