A layer one network is a network that acts as infrastructure for other applications, protocols, and networks to build on top of a public decentralized layer one network's primary characteristic is its consensus mechanism. Different consensus mechanisms provide different levels of speed, security, and throughput.
There are just as many similarities as differences between Layer 1s and Layer 2s. Layer 1 systems are typically decentralized networks that use a consensus mechanism such as POS or POW to update their Ledger. Layer twos can be decentralized or centralized, but they often bypass the mainnets consensus mechanism In an effort to increase transaction speeds and reduce transaction costs or gas fees.
One way to look at Layer 1 versus Layer 2 is by thinking of the Internet as a whole. If the Internet or the World Wide Web is Layer 1, where everything takes place, then Layer 2 would be the web browser you use. That is a gross oversimplification; however, it is the easiest way to view it. If you use the open systems interconnection (OSI) for reference, a Layer 1 could be levels one through six, whereas a Layer 2 could be seen as levels three through seven. The OSI is a conceptual model created by the International Organization for Standardization which enables systems to communicate using standard protocols. While not the same as blockchain, it gives you an idea of how similar Layer 1s and Layer 2s are to each other.
Just as there are several versions or types of layer 1s (Bitcoin, Ethereum, Cardano), there are multiple types of layer 2s (Polygon, Arbiritum). Ultimately, Layer 1 is intended to be the backbone of the user experience. A Layer 2 solution is intended to improve the Layer 1 network by giving a solution to a problem that is believed to exist on Layer 1 for the user. Where Layer 1 is intended to be the backbone of the user experience.