While MetaMask has its own built-in Swaps tool, there's a whole ecosystem out there of third-party token swapping platforms. This article aims to point out some frequent issues that users have when swapping tokens, and serve as a non-definitive directory to third-party swap platform support services. MetaMask is not affiliated with any of these organizations.
Keep in mind that some popular exchanges are not companies in the traditional sense, and may not offer official customer support. Exercise extreme caution if you are engaging with a product through a channel such as Discord or Twitter.
Most importantly: NEVER GIVE ANYONE YOUR SECRET RECOVERY PHRASE.
Third-Party Swap Support Services
DEXs vs CEXs
Decentralized Exchanges, or DEXs, are closer to something like a public service than a business. The question of "how decentralized" they are is a subject of debate, but the core reality is: a DEX is an open software project, whose code lives "in the open" on the blockchain, and with which users can interact in a dependable way, based on how the protocol is programmed. Examples of DEXs include Uniswap, Quickswap and SushiSwap, amongst many, many others.
A Centralized Exchange, on the other hand, is something more akin to an investment bank. A CEX often serves as a "fiat onramp/offramp", integrating with a nationally-issued fiat currency, and allowing users to interact with cryptocurrencies in a more traditional, bank-like environment. CEXs often report users' account and balance information to tax authorities, whereas while interacting with a DEX, this may be left to the responsibility of the user; for more information on tax responsibility, see here.
Price impact is a phenomenon often experienced by users participating in DeFi. For more details, see here.