What should a subnet mask look like?

Could you elaborate on the expected format of a subnet mask, specifically highlighting its key characteristics? I'm curious to understand the structure and purpose of this network parameter in the context of IP addressing. How many octets does it typically comprise? And how does it influence the division of an IP address into network and host portions? Additionally, could you provide an example of a valid subnet mask and explain how it is determined? Understanding this fundamental aspect of networking is crucial for my current project, so a detailed and concise explanation would be greatly appreciated.