Lithium batteries are the new and upcoming, Battery Tender makes good quality lithium batteries, look for a BTL14A240 or Mighty Max ML-LI14A240C.
Lead Acid batteries are old faithfuls and is currently in your stock Maddog. For a good decent replacement look up a Mighty Max YTX-9BS.
CCA = Cold Cranking Amps, which breaks down to if you have a bigger motor than stock you may want a battery with more CCA to get it started easier.
Tenders will increase the life of your battery by making sure they’re always fully charged and ready to go when you want to ride.