First, they belong to completely different classes. Spiders are arachnids which include scorpions, ticks, and mites, among others; reptiles include animals who rely on the outside temperature to regulate their own body temperature...snakes, lizards, turtles, alligators and crocodiles, etc.
Reptiles typically have four legs, although most species of snakes are legless. They also reproduce by internal fertilization.
Arachnids' bodies consist of paired, jointed appendages , a hardened exoskeleton, a segmented body, and a well-developed head. Spiders have spinnerets which allow them to spin their webs in order to catch their prey to eat.
Both are found worldwide and are often misunderstood and feared for unfounded reasons. They are both necessary to the ecosystem.