Products are easier to reason about when you think of them as functions.
They transform an input situation into an output situation.
This lets you describe what the product does as a transformation of the user's circumstance instead of a bundle of features.
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Ryan Singer: Products Are Functions
Consider Basecamp. What is it? A central shared message board? A to-do list manager? Those things describe the solution — its capabilities. But what does Basecamp do for its customers?
Recall the definition of a function: some input gets transformed into an output. You have a function f(). You run x through it and you get f(x) = y.
The user starts in some circumstance x. Whatever product or solution they apply is a function f(). Plugging in the product/service/solution to that circumstance f(x) produces a result: y.
Consider the situation before someone buys Basecamp: