Sterling Silver is made from 925 parts of silver to every 1000 - or 92.5%. It's sometimes stamped 925. The remaining 7.5% is mostly copper. The copper gives it strength, so it's suitable for use in items that will be subjected to stress, such as ear wires or clasps. The disadvantage is that copper reacts more readily with sulphur in the air, which causes tarnishing.
Fine Silver is made from 999 parts of silver to every 1000 - or 99.9%. In other words, it's pure silver. This means that it's softer than sterling silver and is not suitable for making clasps or ear wires. However, the up side is that it doesn't tarnish as readily. The Fine Silver I use is a recycled product and is reclaimed from circuit boards which makes it eco-friendly.
I only work with Sterling Silver or Fine Silver. All the catches, chains, ear wires and so on are made from Sterling Silver, while the decorative components such as the hearts and stars are made from Fine Silver.
