I named my jewelry business after the stamping you find on sterling silver pieces- 925.
Silver is a soft metal. If you were to use 100 percent pure silver it would not hold a form well, other metals (alloys) are added to strengthen the durability.
Sterling silver is 92.5% silver with 7.5% alloy.
Some people mark their pieces with a 'sterling' stamp.
Its just a personal preference.
I'm into numbers, but I'm not a math person- go figure.