Short answer: A DPPO lets you see any dentist (lower cost in-network, with out-of-network coverage) and usually has a deductible and annual maximum. A DHMO assigns you to a network dentist, has no annual maximum, and uses set copays; lower cost but less flexibility.
Dental plans mirror medical plan types. A Dental PPO (DPPO) gives you freedom to see any dentist, pays more when you stay in network, includes out-of-network benefits, and typically has a deductible plus an annual maximum benefit. A Dental HMO (DHMO) requires you to use a contracted network dentist (and get referrals for specialists), but in exchange offers lower premiums, fixed copays per procedure, and usually no annual maximum. DPPOs suit people who want provider choice; DHMOs suit those prioritizing low, predictable cost.