Below is a simple explanation of all the settings for the array modifier. For technical terms and explanation see the Blender documentation.
Three different settings to affect how long your array is
Makes the array as long as a specified curve.
Pro-Tip: Add a curve modifier under the array modifier to have the array follow the curve. Now you can easily see that it is the same length as the curve.
Makes the array as long as the specified length (in Blender Units)
Duplicates the object along the array equal to the count number.
Offsets the array based off of Blender units.
Offsets the array based on the length of the object being offset.
Offsets the array based on the specified objects input.
Protip: Create and empty for this and then try rotating or scaling to see how it affects the array.
Will combine/merge vertices of one object to another if they are within the specified distance.
Similar to above, but only the first and last object will be merged. This is good for circular arrays.
Controls the distance at which it will be merged.
Set and an object to be the starting object of an array and an object to be the ending object of an array.