National Electrical Code 2014
Chapter 8 Communications Systems
Article 800 Communications Circuits
I. General
800.24 Mechanical Execution of Work. Communications circuits and equipment shall be installed in a neat and workmanlike manner. Cables installed exposed on the surface of ceilings and sidewalls shall be supported by the building structure in such a manner that the cable will not be damaged by normal building use. Such cables shall be secured by hardware, including straps, staples, cable ties, hangers, or similar fittings designed and installed so as not to damage the cable. The installation shall also conform to 300.4(D) and 300.11.
Chapter 3 Wiring Methods and Materials
Article 300 Wiring Methods
I. General Requirements
300.4 Protection Against Physical Damage.
(D) Cables and Raceways Parallel to Framing Members and Furring Strips. In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed parallel to framing members, such as joists, rafters, or studs, or is installed parallel to furring strips, the cable or raceway shall be installed and supported so that the nearest outside surface of the cable or raceway is not less than 32 mm (11⁄4 in.) from the nearest edge of the framing member or furring strips where nails or screws are likely to penetrate. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by nails or screws by a steel plate, sleeve, or equivalent at least 1.6 mm (1⁄16 in.) thick.
300.11 Securing and Supporting.
Please see NEC for full text
So basically, you're going to want to protect and support the cables in a similar manner to any other cables. Curiously, the code didn't mention 300.4(A) Cables and Raceways Through Wood Members.. Though I'd likely still follow that section if I had to run communication cables through wood members.
If you run the cable through conduit, you'll have to follow the code for protection and support (fill requirements do not need to be met, per 800.110(B).
If you run through a drop ceiling, you'll have to install support wires for the cable, or support it in another approved manner. You can't support the cable using the ceiling grid, or support wires.
Here's a great document from Mike Holt that explains Article 800 (with images).
Introduction to Article 800 - Communication Circuits (PDF)
Best Answer
Cat6 is capable of very high speeds (by today's standards; hi 2021!), but only within a bunch of additional constraints over lower-speed cable.
Especially difficult is terminating the cable. When you untwist enough to terminate, it's easy to expose yourself to interference. Some people buy long, pre-terminated Cat6 and pull that, instead of pulling bulk cable, for this very reason. You'll need larger conduit if you take this approach.
Another difficulty is strains during the pull. If you tug hard to get around a corner, you can damage the cable enough to stop top speeds from working. Limiting the number of elbows, wide sweeps, conduit bodies, larger conduit, and having someone push while you pull all help.
You may decide that you don't need 10G speeds. In that case, save the money and pull Cat5e. Since you're running conduit, it's easy to upgrade later if you change your mind. It's a good idea to leave a pull string, but if you forget, just suck a new one through with a shop vac.
If there is any EMF in the same trench, a shielded cable will help. Ground the shielding at both ends.
If you are sure you need maximum speed, consider using fiber. You'd put a fiber-to-ethernet converter on each end. Very long runs and EMF are non-issues. Cost scales well with distance.
To make sure I've covered your questions:
Solid conductors, since these cables will be immobile. (Stranded is for wires that move often, like from your laptop to the wall.
Shielded is good for protecting against EMF, especially for long runs or if there is a power cable in the same trench.
Gel-filled cable protects against water intrusion that could cause corrosion.
Direct Burial cable has a tough jacket that can tolerate abrasion underground. If you're in conduit this matters less, but you might choose it anyway.
UV-resistant jacket is important if your wire comes out of the PVC where sunlight can get.
Yes, and this is a good idea. Schedule 40 gray is the normal choice around here.
I dunno about that.
You can run multiple low-voltage cables through the same conduit if you want, but don't put power cable in the same conduit as low-voltage. Use a large enough conduit that you can pull additional cables later.
Many, but I can't think of any more.
EDIT: Since power will be in the same trench, I recommend staking the conduits to their respective sides before filling, to make sure they keep a good foot space between them. I wish I had known to do that!
For future-proofing, consider installing an additional conduit. A contractor friend likes to install an extra 4" conduit, which can be used for a lot of stuff. Other people think he's nuts. I think they're both right. :-)