There are two distinct pre-employment processes that future employers may use to verify your background:

The first type would be a Reference Check. The References would be provided by you. The second process is a background check, often conducted on the employer's behalf by an outside firm. These services will simply verify
your past employment (the number of years they investigate will vary depending on your future employer's needs). They may also verify your places of residence, credit history, and whatever else their contract with your prospective employer specifies. An agency conducting a background check will definitely contact your past employer whether or not you give the prospective employer a person's name there. However, they tend not to get into
reference-checking type questions, sticking for the most part to the name-rank-serial number type info that will fulfill their obligation to the prospective employer, their client.

A cautious employer will only confirm your name, dates worked, positions held and salary if authorized by you. A less cautious employer can basically tell the prospective employer anything, so long as it true - or, if not true, honestly believed by the former employer. In Washington, there is a statute that forbids "blacklisting," which means purposely and with malice trying to block a former employee from obtaining employment elsewhere. Qualified employment lawyers can be found at www.welaweb.org - please do not rely on these necessarily incomplete answers because most of the time, as here, the questioner gives far too little information - and also because it is not our intent to create an attorney/client relationship by participating on this website. Good luck.