There are three fundamental units we'll deal with in this course: length, time, and mass. We'll see shortly how other quantities of physical interest can be made out of these three units.
You already have a concept of units. When someone asks the rather rude question ``how tall are you?''. You probably have an idea of what the answer is, in some units. If you live in the U.S. you'll know the answer in feet. If you live in most other parts of the world, you'll know the answer in meters. Scientists have tended to adopt the units of meters. There's no particularly good reason for this, the only thing that's good is that it's quite standard. When a scientist in Zimbabwe says something is 1.06 meters, scientists in Moscow will know exactly what is meant. If everyone had different measures of length, then it would make life far too complicated.
Same with time. There is nothing fundamental about the notion of a second, its roughly one sixtieth of one sixtieth of one twenty fourth of the time it take the earth to spin around its axis once. It's completely historical. Nowadays, scientists have adopted some standard for what a second is to avoid any misunderstandings. You have a pretty good intuitive understanding of a second.
Mass is something that can be a little trickier to understand. When you run around on the moon, you'll notice that you seem to be able to jump higher. You actually weigh less. Does that mean to say you have less mass? Well actually no. You have the same mass (for all you afficianados of relativity out there, keep a lid on it!). You have the same mass in either case. We'll talk more about mass when we discuss Newton's laws. Mass isn't measured in pounds, that measures a force, mass is normally measured in kilograms. If you like working with feet instead of meters, then you might also want to work with a unit of mass called a ``slug'' but they're slimy and I wouldn't advice it. I'm also tired of all these UCSC slug jokes so I won't make any more stupid puns for the moment.
We'll deal mostly here with ``SI'' units, kilograms (kg), meters (m), and seconds (s).