Hi Bob & thanks Burr!

re: Trim - trim is a surface modeling operation. When you trim an object it only cuts the surface skin of the object and will not leave pieces of the cutting object behind. Usually you will want to use Trim if you want to work at a surface level and plan to join some various surface bits together into a solid at the end.

The boolean functions on the other hand are oriented towards working with solid volumes, and they will leave pieces of the cutting objects behind to keep the objects as a solid. You can do booleans between solids and curves - the curves get automatically extruded so for example doing a boolean between a solid and a line is the equivalent of cutting it with a plane extruded from that line.

Check out this post for some more description on the difference between Trim and Booleans for cutting things:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3883.3

By the way, if you do use Trim to cut a solid with a line, you will end up an open surface object that has a planar opening in it. But you can then make that object a solid by selecting it and running the Construct > Planar command. That will construct a planar surface that fills in the hole and then that surface is joined in with the others, and as long as you didn't have any other non-planar openings in it then that will be a solid.

For example here is a sphere that has been trimmed with a line:



If you select it and run the Construct > Planar command, it will build a cap for the open edge there and you will then have a solid that looks like this:



But if that's what you wanted to make it is more convenient to use Boolean difference to cut the sphere rather than Trim since it will make it a solid piece like that without needing to anything else to it.

Hope this helps!

- Michael