For the purposes of thinking about language, there is an important distinction to make between three sorts of comparison. The first two are imaginative comparisons and literal comparisons. (There are also degrees of comparison in grammar, that is, formations like "big, bigger, biggest".)
Literal comparisons are the simplest. They are the comparisons we use to describe or explain things. "A bus," a mother might say to her child, "is like a car, only bigger." A teenager might have to tell a grandparent that an i-pod is like a record-player, only smaller. An old-fashioned history teacher said that he couldn't see the sense in fixing a typewriter to a television screen -- he thought that the first computer he had seen was like that.
Somewhere between the literal comparison and the imaginative comparison comes the analogy. An analogy is a comparison that, without being creatively figurative, tries to explain or describe one thing in terms of another. This is to help the hearer understand or visualise the phenomenon being described. Analogies are often used to indicate size: people describe an agricultural field, or a building site, by saying "It's the size of [a number of] football pitches." One can describe archaeological time by making an analogy between the whole of the earth's lifetime and an ordinary day: "The first humans appeared well after 11 o'clock at night, and written history occupies only a minute." The adjective to describe this is analogous. (the 'g' in analology is 'soft', like the sound at the end of "fridge". The 'g' in analogous is 'hard', like the 'g' in "get").