As the jury instruction says, "there are two types of evidence from which you may properly find the truth as to the facts of a case. One is direct evidence. The other is circumstantial evidence, that is, the proof of facts from which other facts may reasonably be inferred."
Prosecutors are quick to point out "the law makes no distinction between direct and circumstantial evidence," although prior to 1973 in Colorado (and most jurisdictions) there was a distinction. That year the Colorado Supreme Court (following then recent federal case law) changed the law, although two justices disapproved dropping the "long established … protection against the conviction of an innocent person upon wholly circumstantial evidence." People v. Bennett, 515 P.2d 466, 471 (Colo. 1973) (an early drug war case when courts unfortunately began equating justice with convictions).
Prosecutors often give the example of footprints in the snow as circumstantial evidence someone has walked by. Direct evidence of that fact of course would be an eyewitness.
Defense attorneys try to explain that circumstantial evidence is not as good as direct evidence. This is especially difficult to do since jurors reasonably think the lack of distinction as instructed makes both types of evidence equally good, when really it is supposed to mean both types are equally good or bad.
Take the footprint example - they don't necessarily tell you when, or who, or why someone walked by - or if they were going forward or backwards, carrying something or someone, whether someone followed in them, etc., etc. An eyewitness could tell you more of those things.
To experienced defense attorneys, the more a prosecutor talks about circumstantial evidence the weaker the prosecution's case.
Remember this: circumstantial evidence is used to prove the existence of Bigfoot (footprints), the Easter Bunny (chocolate eggs in your backyard) AND Santa Claus (presents under the tree). It is also used to convict the innocent.
So, when you hear someone talking about circumstantial evidence proving, or confirming or suggesting something, be wary.