Inheritance from Revocable Trust

My grandfather passed away years ago and any assets set to be distributed to beneficiaries went into his revocable trust.  The trust is not set to be broken up and transferred to the beneficiaries until a trigger date occurs.  When that date occurs and the trust pieces are transferred to the beneficiaries, are the assets transferred within it considered taxable income?  We are having trouble understanding whether it's considered inheritance or not since the transfer is coming from the trust and not an estate.  If some of the assets are the form of investment portfolios, are the earnings on those taxable from the beneficiaries perspective from the time the investment was bought within the trust or only from the time the transfer occurred to the beneficiary?