Read this:
http://www.apple.com/batteries/notebooks.html
It's a good article. The batteries are "smart", meaning that they are self-protective (they won't over or under charge). They have a processor that tracks a number of things. To make sure that your getting the best-estimated battery power/time remaining/% of charge, you need to drain the battery until the computer puts it's self to sleep and can't be woken again. This causes the battery to calibrate its self. Do this once a month.
Neat battery widget.
X-ChargeThis little app is great. It gives information in the DOC and also give detailed stats as well as a charge and discharge graph over time. This is always running on my computer. I have it set to load on startup.
A neat little app that gives more information.
To see how many cycles and how what the capacity of your battery is:
Finder ->About This Mac-->Click More Info-->Contents->Hardware->Power>Battery info.
Write down or note the Full and Remaining Capacity number somewhere. Over time the Full Capacity number will drop slowly with time and charge cycles. The Remaining Capacity number will drop as the battery is discharged, but over time its maximum (Full Capacity) will decrease. Note the number of Cycles to see how many times you have completly charged and discharged (even incrementaly) the battery. This is just for kicks, but I like to know how my batteries degrade over time.