Simple trusts distribute all their income in any year to non-charitable beneficiaries and does not distributed any of the principal assets of the trust.
Complex trusts are are anything that isn't a simple trust, any of these: one that retains income, one that has charitable entities as beneficiaries, or one that distributes any of the principal assets (called the corpus).
It's primarily a tax distinction rather than a statement of how incomprehesible it is.