Crystal and glass are the same thing. Crystal is glass that contains lead. When lightly tapped it has a sharp ring compared to plain glass.
If you tap it lightly with your fingernail and it rings, it is crystal.
If a decanter is lead crystal, there should be a mark on the bottom of the item. You can read more about lead crystal at http://www.wisegeek.com/what-is-lead-crystal.htm.
I could not find any information about a lead crystal decanter. It's possible it is just not listed on their website.
Traditionally lead crystal if it is high quality. Lead is no longer used due to a possible health risk - just high quality pure crystal glass.
Lead crystal will leach a small amount of lead into foods placed in the container. For short durations, such as serving, they are safe. However, lead crystal is unsuitable for long term storage, or for use by pregnant women or small children due to lead poisoning concerns.
The best way to find out how much your antique crystal decanter set is worth, is by taking it to your nearest local antique dealer. They should be able to tell you how much it is worth and other information about the set.
A crystal container for alcohol is called a decanter.
Acidic liquids, like wine, will absorb lead from leaded crystal. However, the process is so slow that you shouldn't worry about sipping a glass of wine over an hour or two or serving from a decanter that you filled with wine that very night. The real danger occurs when you store beverages in lead crystal decanters for long periods of time (I'm rather sure it takes at least days, but potentially longer, for lead levels to reach a dangerous amount). The simple rule of thumb is to add the beverage to the decanter on the day you plan to use it, and never leave any leftover liquid in the decanter for storage. Lead is bad for anyone, but kids are especially affected by it. All the recommendations I've read say to be extra safe and simply avoid letting kids drink anything from lead crystal.
A crystal decanter is usually used for something that has to be aged. An example of alcohol would be malt scotch.
A waterford crystal decanter is a jug type to hold water or any liquid to drink. It is made from crystal by the Waterford company. These are considered high end collector items and can be quite expensive.
I can't tell if you mean "crystal" as in "made of a crystalline material" or "crystal" as in "made of lead crystal, which is not technically a crystal at all". Take it to a jeweler. There's not any simple foolproof way a layman could tell.
A Baccarat decanter is a decanter made of Baccarat crystal and is used to decant liquids such as whiskey or cognac. Baccarat decanters can be found in many beautiful shapes and styles.
Amazon.com carries a good selection of crystal decanter sets, ranging in price from $12.40 to $89.99. Another great option is eBay.