Classification is the process of putting things into groups. It is a skill needed in many areas of life, not only in science.

 You use it every day.

Classification is used in the shops. Eg the sweets are in a different area to the savoury foods. Next time you walk round a supermarket, think about the way products are grouped. Biscuits are all in the same aisle, with the savoury biscuits separated from the sweet and chocolate biscuits. Canned items are usually together, but the canned fruit is separated from the baked beans and soups, and fresh fruit is organised neatly in crates.

Music you download from your iPod is classifies in different ways because you have classic music rock music and pop music.

Groups change.

Plants and animals are often grouped in different ways too. For example, the grass making up a garden's lawn is looked after and encouraged to grow. However, if the grass escapes from the lawn area and into the flower beds, the grass is now considered to be a pest and is dug out or even poisoned. The plant that isn't wanted in one place is a weed.  

here are some pictures.

Imagine the following to girls. There are two easily observed differences between these girls. Only one has a pony tail, and therefore long hair. The other has short hair and no pony tail. One has blue eyes and the other brown eyes. You can use any of these differences to separate them in a key; however, only one of these differences would give you a strong key. The girl with the ponytail and long hair could wear her hair down or cut it short. The other girl could grow her hair longer. One difference will always be the same- the eye colour. It is the difference in eye colour that will give you a strong key.