Electrical wiring under deck
Yes, PVC conduit is nice stuff.  Make sure to keep water from entering it, or it may carry water to places you don't want water. (that said, all outdoor conduit is presumed to be 100% full of water all the time, so you must use wet-rated wire). 

You'll be using single wires in conduit, just like most commercial buildings.  You need wire marked for wet locations.  There are many grades of wire, but what I find on store shelves is THHN or THWN - the latter has the "W" wet rating.  The cost difference is so small that many places only stock THWN -- but check, don't take any THHN.  

You can choose solid or stranded wire, stranded is a lot easier to work with, but sometimes more difficult to terminate.  

You cannot shuck down Romex wire and use the white and black wires, because the interior wires are not labeled.  Shucking down to bare wire for bare grounds is fine.  

You will need at least 3 rolls of wire.  It's most economical in 500ft rolls.


Green or bare for ground wires (if you use them)
White or gray wire for neutral (I recommend white, you can use gray to distinguish a neutral from a different circuit.) 
Any color not mentioned above, for your hots.  Most people go black, but you are welcome to install a rainbow of colors if you like, the only rule is you must be reasonably consistent within your installation.  So if you use orange for switched-hot for lighting, stick with that. 


You will need to run a ground wire with plastic conduit.  I recommend you run a ground wire with metal conduit - in the great outdoors, that stuff will rust, even if it's galvanized, which will eventually cause grounding problems. 

If you have metal (commonly aluminum) boxes in your system, there will be a hole already tapped 10-32 for a ground screw.  Attach a pigtail of solid wire to that screw, and wire-nut it to your other grounds.  I use stranded wire myself, and a crimp-on ring terminal. 

In your photo, that guy needs more support for some of those pipes, especially the long run on the left.   PVC needs support at closer spacing than steel EMT. 

Don't even think about buying PVC at the big-box store or any national-chain seller, because they won't have all the parts.  They'll have brand X boxes and brand Y box covers, and no, they don't play well together.  BTDT.   Rather than have to go to the electrical supply house 3 towns over for the missing bits, create a relationship with the electrical supply house in your town and use the system they sell and fully stock. For one thing, it'll be cheaper - big-box creates the illusion of cheap, but nails you on all but the most mainstream.