Sign are made with cut vinyl or digitally printed vinyl and the vinyl can be applied to the front of opaque materials and the backside of clear
materials (acrylic or polycarbonate). Here is a list of available substrates:
Aluminum Composite Material (ACM)
Aluminum sheets with a solid plastic core, this material is replacing MDO Plywood as the most common choice for signs
This is a very stable flat material that is great for moderately economical signs. 3mm being the most common PVC sheeting is available in many thicknesses from 1mm - 20mm mostly in white for thicknesses other than 3mm.
This is the fluted material you tend to see everywhere. A very popular economical option. 4mm white is the most common style but other thicknesses and colors are available
Acrylic substrates are nice when a solid material is needed. The clear can be backpainted which gives a glossy look which can be nice, also there is such a wide variety of colors and finishes available.