Just to back up ABerrant's post:
Police forces used to be allowed to 'step down' cautions after a set period of time but, following a court ruling, cautions are now retained on file for life (or, for pedants, until the person's 100th birthday).
So a caution will always show up on any CRB check at 'Standard' or 'Enhanced' level. Those are the only two levels of check available in England and Wales, but they are only available to employers and organisations when the employee (or volunteer) will be working closely with, or alongside, children or vulnerable adults (or when the work involves national security or the administration of justice). Other employers have no rights to access criminal records.
In Scotland and Northern Ireland employers can also apply for a criminal record check at 'Basic' level. (Such checks are available to all employers, whereas 'Standard' and 'Enhanced' checks are again restricted to certain occupations or voluntary activities). 'Basic' checks do not show cautions or spent convictions.
You can't be cautioned unless you agree to accept that caution (which is effectively an admission of guilt). Once you've agreed to accept the caution the chances of getting it revoked are extremely low (unless, for example, you could show that you didn't have the mental capacity to make an informed decision about whether or not to accept the caution).
Chris