| Unity refers to the fact that all living organisms are descended from a common ancestor and share the same genetic code and many other commonalities. All organisms are made of cells that have a similar structure. All have DNA as their genetic material. And so on.
The diversity of life is just what it sounds like. Despite all organisms being related to one another, the Earth hosts millions of different species, something like 10 million or 30 million. No one knows the exact number.
Over the history of life on Earth, there have been even more species than that, since 99% of species that have ever existed are now extinct. Life just keeps changing and evolving. Species come and go in the same way that generations of a particular species live, reproduce and die over time.
PS: navy's comment about the central dogma is factually incorrect. No offense, I hope. The central dogma says that information flows from dna to mRNA to protein, and in fact, nowadays, we know that it's often more complicated than that. Information can flow differently in some circumstances. | 
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