Class defines the speed. Class 10 is faster than class 6. Unsure of the absolute relation between the classes though, but higher is faster. All the bigger brands (Sandisk, Kingston, Sony etc) should be fine, the only thing to watch out for are imitations on eebay and fake ones (where you buy a 16GB, it displays 16GBs when you check the properties, but then only actually holds 2GB).