Camshaft
The camshaft in an engine is responsable for the opening and closing of the engines valves. It is the first member of an engines valve train. Along the length of the shaft are a series of lobes each at a different angle of a circle (360°). These angles are important as they correspond to the exact intake or exhaust valve that should open or close at the appropriate time in the four-stroke cycle.
Each cylinder in a four stroke engine has two poppet valves (valves with springs to force them closed). 
There are different configurations of lobes and many of todays engines offer dual over heas cam (two cams to each cylinder head) engines. There is always going to be an intake and an exhaust lobe dedicated to each cylinder. There is one lobe for the intake valve and one for the exhaust valve. As this camshaft turns these lobes open and close the valves in exact timing with the engines crankshaft by sprockets driven by a timing belt, chain, or fixed gears.
The lobes on a camshaft are not in direct contact with the engines valves. Depending on the design of the engine: whether the cam resides inside of the engine block or an overhead camshaft design, different methods will be used to open and close the cylinder valves.
A camshaft placed in the engine block will use a lifter and push rod to move a rocker arm as a "lever" to open the valves. An overhead engine will use a tappet bucket in direct contact with the cam lobe to open the respective valves. Many overhead cam engines use a rocker arm assembly. These are rocker arms affixed to a shaft that is typically removed as a one piece assembly. Overhead camshafts are a much simpler design and are easier to replace.
There are three measurements or values that should be considered when replacing a camshaft:
Valve Lift: Valve lift is the distance the valve is opened into the engines cylinder. This is the result of the valve train. In a camshaft in-the-block engine this includes the camshafts lobe, the push rod, and the characteristics of the rocker arm ratio (1.5:1 or 1.6:1) etc. In an overhead cam engine this involves the lift (distance from the center of the shaft) of the camshaft lobe and the tappet bucket.
Camshaft Duration: This is the amount of time the valve is open and off its seat. This measurement is taken in crankshaft degrees. Simply put this is the time the camshaft holds the valve open in a cylinder. When we say measured in crankshaft degrees we are saying how much of the complete circle a crankshaft turns while the valve is open. A circle is 360 degrees and the valve only stays open a small portion of that circle. It's common to have a 220 degree camshaft duration measured at .050 in. or fifty thousands. If you were to build a race car you would want more duration which increases the flow through the engine. This would have a negative affect on the engines idle do to valve overlap (both valves remain open at the same time resulting in a loss of some cylinder compression).
LSA or Lobe Separation Angle: This is the distance between the two lobes in each cylinder. This measurement will affect the idle and high RPM characteristics. A wider angle will give an engine a better idle while a narrower angle will increase the higher RPM performance. This is measured in degrees on the camshaft not the crankshaft.