Well if a piece weighs more 7.78 grams it needs to be hallmarked otherwise it cannot be classed as "sterling Silver" (or any other silver, we will come onto that in a min!)
If your jewellery is hallmarked you will usually see 3 or more little marks
The sponsors mark is your initials or company initials in one of many different shapes. Mine is ZJ for Zazu Jewellery in a shield. Each sponsors mark is unique and when you apply you have to put a range down incase its already taken.
The standard mark states what purity of the metal is. In silver there are 3 main marks that you will see.
925 - Sterling Silver meaning there are 925 parts in 1000 of silver
958 - Britannia Silver
999 - Fine Silver, which is a lot softer than sterling and is the metal that is produced from Precious Metal Clay
There are four Assay Offices in the country and each has their own mark.
You may also have a date stamp although this isn't mandatory.