How is perjury different from lying?

Perjury is different because lying has three different forms which is rationalizing, white lies, and lying by omission. These all involve ways of lying to a person or yourself while perjury isn't exactly lying because what it is, is forging a person's signature. The way this would most likely relate to a lie would be by omission because you aren't telling the person you forged the signature to that the signature isn't legit. How is perjury different from making false statements? To commit perjury, you have to be under oath, and you have to knowingly fib about something that's relevant to the case at hand. (Your statement must also be literally false—lies of omission don't count.)  

For instance, a witness who lies about his whereabouts during the crime is committing perjury. If he lies about how long he has worked for his employer may not be perjury under the law, unless it is somehow material to the topic of the legal matter.