In summary.txt what's the difference between length and bases?

[nuno5645]

chrom length bases mean min max
chr13 115169878 8508921 0.07 0 8833
chr13_region 13390 10009503 747.54 0 8833
chr17 81195210 5713210 0.07 0 8689
chr17_region 8316 6078518 730.94 0 8689
total 196365088 14222131 0.07 0 8833
total_region 21706 16088021 741.18 0 8833

so basically i want to get the number of bases and mapped bases so i can add to my quality metrics report.
The problem is that i am at a loss at which variables i should look at