1- D-RAN stands for “Distributed RAN”
D-RAN is a classical setup where RRU (Remote Radio Unit) and BBU (BaseBand Unit) are co-located at every cell site. They run proprietary applications on specialized hardware. Each radio site with all of its functions are in a single location and connected back to the core network through backhaul.
2- C-RAN (Also CRAN) stands for Centralized RAN or Cloud RAN.
In C-RAN the BBUs from several sites are grouped into a pool and located in one location. This leads to more efficient use of computing resources. but what is important to understand is that we still need software and hardware to be coming from a single vendor.
3- vRAN called also Virtualized Ran or Centralized Ran
vRAN and C-RAN are very similar. Only difference is that C-RAN uses proprietary hardware while vRAN deploys on top of COTS servers that you can find typically into a Datacenter.
4- Then finally OpenRAN
The O-RAN alliance made a great job creating a specification for open fronthaul, mid-haul and backhaul interfaces.
This means that finally you can mix and match different vendors, and use different approaches without vendor lock-in.