A different philosophy
Most ecosystems try to get everyone onto one chain (or one chain plus its rollups). Cosmos takes the opposite view: let every application have its own purpose-built blockchain ("app-chain"), and connect them all so value and data flow freely. It's less a single network than a movement and a toolkit for building interoperable sovereign chains.
The key technologies
Two things define Cosmos. The Cosmos SDK is a framework that makes it dramatically easier to build a custom blockchain, so teams can launch their own chain tailored to their app rather than renting space on someone else's. And IBC (the Inter-Blockchain Communication protocol) is a standard that lets these independent chains send tokens and messages to each other securely — the connective tissue of the "internet of blockchains."
What's built on it
Many significant chains are built with Cosmos technology and connected via IBC, including DeFi-focused chains, the dYdX derivatives chain, Celestia (a modular data-availability network), Injective, and Osmosis (a leading interchain DEX). The ecosystem is a web of specialised sovereign chains rather than a single monolithic platform.
Strengths and trade-offs
Cosmos's strengths are sovereignty (each chain controls its own rules and isn't constrained by a shared base layer), flexibility, and genuine interoperability via IBC. The trade-offs are that each app-chain must bootstrap its own security and validators (harder than inheriting it from a big base layer), and that liquidity and users are spread across many chains. It's a powerful vision that asks more of its builders.