State of The Estate
While terms like "Reserve" or "Old Vines" have no rules behind them, the use of "Estate" on wine is regulated by the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Three Requirements to Fit Definition of Estate Wine
2. The grapes must come from vineyards owned or controlled by the winery. If you own a winery and purchase fruit from another vineyard owner, you must have control over the vineyard. This would include decisions on what grapes to plant, irrigation, cultivation and other important decisions. If you don't control the vineyard, you can't put "estate" on the bottle.
3. The wine must have been produced, from the initial crush to bottling, in a continuous process without leaving the winery's premises.
Usually estate wines cost more because of the extra care and effort that goes into production of the wine. This doesn't mean that non-estate wines are bad. Some wines are labelled "California" for example, with grapes from several different wine growing areas -- but they can still produce exceptional wines.
What About Reserve Wine?
By the way, although the term "Reserve" isn't regulated if you are buying from a respected winery, you should feel pretty confident that there is something special about the wine. "Reserve wines" are typically considered the winery's best or upper echelon wines and good wineries try to live up to that standard. With struggling or unknown wine producers: buyer beware.