The offence of fraud was created by the Fraud Act 2006. It replaced other more specific offences, though some offences involving fraud still exist alongside it, such as false accounting.
Fraud can be committed by making an untrue or misleading statement, or saying nothing when there was a legal duty to say something. This is with the intention of obtaining a gain in money or other property or causing a loss, or risk of loss, to the victim. It can also be committed by abusing a position of trust.
To be convicted of fraud you must have acted dishonestly, as explained below.
Dishonesty is an important element of theft and other offences besides fraud. For example, theft is not just taking someone else's property, but the taking must be "dishonest".
Usually it will be obvious from what you actually did that proves you must have been dishonest, and this does not take much proving. However, in a borderline case where dishonesty is in doubt, the prosecution would have to show that what you did was dishonest by the standards of reasonable and honest people, and that you knew it was.