You will often see silver coins, rounds and bars being sold as weighing one or more 'ounces'. 
But sometimes the seller doesn't mention which type of 'ounce' they mean. 
There are two different types of ounce - the Troy ounce and the Avoir Dupois (AV) ounce. 
Precious metals are usually weighed in Troy ounces. AV ounces are the type that are used for everything else and are type found on your kitchen scales etc.
The way to discover whether an item is a Troy ounce or AV ounce is to ask for the weight in Grams.

1 Troy ounce = 31.103 grams
1 Avoir Dupois ounce = 28.349 grams

As there's a difference of nearly 3 grams, modern kitchen scales are usually accurate enough.

And then there are 'Grains', which the silver content is sometimes stated as.

There are 480 grains in a Troy ounce and 437.5 grains in an AV ounce.

Sometimes you will see silver bars that are apparently silver and apparently weigh 'one ounce' yet have a silver content stated in grains of a lot less than you would expect to see in one ounce.

1 ounce troy = 31.103 grams = 480 grains

1 ounce avoir dupois = 28.349 grams = 437.50 grains

1 gram = 15.43 grains

Other info:
Sterling silver is 925 part in 1000 parts pure silver, ie 92.5% silver.
.999 silver is 999 parts in 1000 parts pure silver, ie 99.9% silver.
.900 silver is 90% silver  and is what pre-1964 USA silver coins are made of.

It is usually copper that makes up the other parts of the weight.