You can use this Capo Chart to determine what the actual sound of a capoed chord will be. For example, let's say you've moved your capo around and find that you can sing a song most comfortably when you play an A chord capoed at the fifth fret. Since you're capoed, what you're playing no longer sounds like an A. What chord are you really playing? Another guitarist could simply place his or her capo at the fifth fret and copy your chords but anyone else playing a non-capoed instrument, say a man dolin or bass, would need more specific information. You can find that information in the Capo Chart.
Look in the chart above and find the A chord in the first column on the left. Now follow over to the right to the column labeled "5." This will tell you that an A chord capoed at the fifth fret will have the actual sound of a D chord. As a rule of thumb, if the song begins, and more importantly ends, on the D chord, you're most likely in the key of D. Anyone playing along with you will have to play in the same actual key of D.
You can also use the Capo Chart to determine other chord/capo combinations for the same key. Let's look at the example above. You've chosen to play an A chord capoed at the fifth fret with an actual sound of a D chord, likely in the key of D. How else could you capo and what chords could you play and still be in the key of D? Start by looking under the different capo position columns (1, 2, 3, etc.) to find the D. Moving from the left, the first we find is under the "2" column, "C" row. That means that if we play a C chord capoed at the second fret, the resulting sound will be a D chord. Another combination is under the "7" column, "G" row. If we play a G chord capoed at the seventh fret, the actual sound will be a D. Using these different chord/capo combinations can give you an great range of interesting timbres on the guitar. When playing with more than one guitarist, try each capoing at a different position.