Technically you do not have to create a cosmos DB and incur costs to test cosmos DB based applications, you could use the local emulator. This means that you do not need an Azure subscription, an actual hosted database or even an internet connection, everything is local to your machine and once ready you can deploy the solution to the cloud.
Cosmos DB falls into the “NoSQL” technology group, call it a buzz word if you like but it is very different to classic Relational Databases such as SQL Server. I never really understood why it was labeled NoSQL, if you build a document based data model via the SQL API you actually use SQL like language to query the JSON (from the collection). Moving on, it is an evolution from what used to exist called Document DB. (Cosmos DB sounds way more aggressive and punchy, right?) Recently I have been using it to store JSON documents (Document based data model) in Azure, as a fully managed service, i.e. PaaS and maybe you will use it one day.