FR4 fiberglass sheet is a fire rated electrical-grade, dielectric fiberglass laminate epoxy resin system combined with a glass fabric substrate.
The abbreviation "FR4" means: F (for flame) and R (for retardant),  # 4 epoxy.
FR4 grades offer excellent chemical resistance, flame ratings (UL94-VO) and electrical properties under dry and humid conditions. FR4 also features high flexural, impact, superior mechanical strength and bond strength at temperatures up to 170°C. 

G10 fiberglass sheet is a composite material made of glass cloth and epoxy resin. It was developed as an aircraft material when it was invented, it can withstand great strength. G-10 is not permeable with water and liquid, and it has the characteristics of insulation, anti-acid and alkali, and the weight is light. 
A continuous woven glass fabric laminated with an epoxy resin. This grade is extremely high in mechanical strength, has low water absorption and dissipation factors and has superior electrical characteristics, which are exhibited over a wide range of temperatures and humidities.