So it's like DNS for GPS coordinates. Neat!
"You shall not use the API for any application that replicates or attempts to replace the essential user experience of the Service or use the API to create a Product which functions substantially the same as the API and offer it for use by third parties. You must not pre-fetch, cache, index, copy, re-utilise, extract or store any what3words Data"
Kind of a big difference. With DNS you only have to pay if you want a domain. I can freely do a DNS query to resolve "flamingmongoosefarts.com", but with What3Words I (might not) be able to freely determine where flaming.mongoose.farts is.