It depends on your class of ticket, SkyMiles level and where you are flying to.
If you purchase a low-fare ticket for domestic travel you are charged $23 for the first bag and $32 for the second bag.
If you purchase an unrestricted Class Y economy ticket there is no charge for up to two bags.
If you are flying first class then there is no charge for up to three bags.
If you are flying International then your first bag is free and the second bag is $50.
If you are a Silver or higher member of SkyMiles then there is no baggage fee for two bags.
If you have Delta Skymiles credit card then there is no charge for the first bag.
For the average vacation flyer then you will be charged $23 for the first bag and $32 for the second.