Dave,
Trim and tilt are 2 different functions yet they are combined together.
A lot of smaller Yams only have "tilt" which is there to enable easy lifting of the engine when not running (or not under power).
Perhaps the easiest definition is to consider the range of movement of your engine from vertical (or thereabouts) out to somewhere approaching horizontal. From engine fuly down position you have perhaps 15 degrees of TRIM when fitted with power trim and tilt. The rest of the movement is TILT.
TRIM adjustments can be made under power to trim the bow of the boat. If you trimmed your engine out into the TILT range and then put the throttle down you will overload the TILT and the engine would force itself down to the outermost TRIM position and stop there.
Most power trim/tilt systems have a large central ram which is the TILT ram and then 2 shorter rams (1 either side). THese are the TRIM rams and are the rams that can handle the load of the engine under power and adjust the engines position.
Cheers