Circumstantial evidence is evidence that relies on an inference to connect it to a conclusion of fact like a ‘fingerprint’ at the scene of a offence/crime. By contrast, direct evidence supports the truth of an assertion directly i.e., without need for any additional evidence or inference.
On its own, circumstantial evidence allows for more than one explanation. Different pieces of circumstantial evidence may be required, so that each corroborates the conclusions drawn from the others. Together, they may more strongly support one particular inference over on another. An explanation involving circumstantial evidence becomes more likely once alternative explanations may have been ruled out.
Circumstantial evidence allows a Trier to infer that a fact exists. 
In criminal law:- the inference is made by the Trier of fact in order to support the truth of an assertion (of guilt or absence of guilt).
Testimony can be direct evidence or it can be circumstantial.
For instance: a witness saying that she saw a defendant stab a victim is providing direct evidence. By contrast, a witness who says that she saw the defendant enter a house, that she heard screaming, and that she saw the defendant leave with a bloody knife gives circumstantial evidence. It is the necessity for inference, and not the obviousness of a conclusion, that determines whether evidence is circumstantial.
Forensic evidence supplied by an expert witness is usually treated as circumstantial evidence.
For instance:- a forensic scientist may provide results of ballistic tests proving that the defendant’s firearm fired the bullets that killed the victim.

Circumstantial evidence is especially important in civil and criminal cases where direct evidence is lacking.