DPCs (Dense Port Concentrators) were the original line cards released for the MX series back around 2006.
They were named so because they were a fixed configuration with high (for the time) port capacity.
Not long after, MPCs (Modular Port Concentrators) were released, which as the name implies were just a carrier card for MICs (Modular Interface Cards) so you could pick and choose your interfaces.
MPCs also used Juniper's Trio ASIC, which gave them much better scale and features than was possible on the older DPCs.
These days, MPCs aren't always modular - some only come in fixed form-factor, however the name has stuck and they still all include one or more Trio ASICs.
DPCs are all end of life