The most common type of map scale is the bar scale or linear scale.
Imagine you just moved into a new house or apartment. You take a trip to the nearest furniture store and buy a few storage shelves to put up so you can display all of your prized possessions. You get the boxes home, open them up, and lay out the pieces of the shelves. You find all of the nuts and bolts you will need and then discover quickly that something is very wrong. There are no instructions in the box! Somehow, the instruction page was left out of your box and you have no idea how to put the shelves together. Without the proper guidance, even a simple task can become impossible.
When reading a map, you need to refer to a set of instructions in order to understand distances in real life. A map scale is a ratio of the distance on a map to the actual distance of the ground. Map scales can usually be found on the outer edge of a map, usually near the map's key (a set of symbols that also make the map easier to understand). Whether the scale is drawn out or written, you can easily calculate distances if you know the ratio.
The map scale on most maps is quite a large ratio. After all, maps are a physical representation of the Earth. In order to have a 1:1 ratio of map to ground, you would have to carry around a map as big as the planet! Pockets are usually not big enough.
Types of Map Scales
There are two main types of map scales: bar and lexical. In a bar scale, which tends to be the most common, the mapmaker has given you a visual guide to use to make distance calculations. You can use any measuring tool or a piece of string to figure out actual distances on a map by directly measuring the map distance and translating it to the bar scale.
A lexical scale is not visual like a bar scale. Instead, the mapmaker gives the distance conversion in words. For example, the map may include the scale written as '1 inch equals 10 miles'. This is generally viewed as less practical and is, therefore, much less common. With this scale, it is also much more common to run into language barriers for speakers of languages other than what the map is written in. This scale works in the same way as the bar scale, but it is just represented differently.