Without getting too technical...
1) One works in tandem with modern DSLRs and includes preflash functions for metering, incorporates advanced microprocessors and usually features low flash triggering voltages,
2) No, it's absolutely possible to use analog flash units with DSLRs. Just be sure to check the flash triggering voltage limit of your camera body, and the flash triggering voltage of the particular flash unit you're interested in (Google). Some advanced systems like the Metz SCA may still give you TTL functions on particular models with the appropriate SCA adapters.