There are five basic units that we shall be concerned with. These are, together with how they are measures in the metric (SI) system, are summarized in the table below.

Probably the first four are familiar to us; the last one, the unit of charge, we shall encounter when we discuss electricity.
It is a good idea to have an intuitive idea of the relative sizes of these units  a person might be 2 m tall, have a mass of 70 kg, etc. However, for some quantities, such as the age of a person, or how far it is to Paris, these fundamental units may be inconvenient due to the large or small numbers involved. For this reason we often will use prefixes attached to these basic units to denote some multiple of them. Some of the more common prefixes are given in the table below.

Here we have also used scientific notation, whereby, for
example, 10^{3} = 10 x 10 x 10 = 1000, and 10^{3} =
1 / ( 10 x 10 x 10 ) = 0.001. Thus, for example, 1 km = 1000 m.
To convert units, one simply multiplies by the relevant
conversion factor, as, for example,
3000 m  =  3000 m x = 3 km. 
12 in  =  12 in x = 30.48 cm 
20 $ CDN  =  20 $ CDN x = 14.8 $ US 