Azure Cosmos DB

Some of the concepts in Azure Cosmos DB:

• Database account is a container for one or more Azure Cosmos DB databases.
• Database is a container for one or more collections
• Collections are like tables in relational databases without a schema. A collection contains documents.
• Documents are an unstructured set of key/value pairs, read and written in JSON format. They are the logical unit and which is roughly equivalent to a relational row. Documents of similar types are organized into collections.
• Partitioning is the distribution and grouping of your data across the underlying resources.
• Index is a catalog of document properties and their values. 
• Request Units (RUs) measures the throughput at which rate data is processed. RU is the amount of CPU, disk I/O, and memory required to read 1 KB of data in 1 second.

Additional insights can be found at How To Design And Query Data In Cosmos DB (c-sharpcorner.com)

Your default choice for new Azure Cosmos DB accounts should be Core (SQL). However, it would help if you also considered the following situations:

If your data is better represented in a graph, the Gremlin (graph) API might be a good choice.

If you already have an existing application or database using one of the other APIs, then the current API might be a better choice for your specific scenario. Using the current API might make it easier to:

• Migrate your application or database to Azure Cosmos DB
• Reuse your existing code with minimal changes
• Leverage the existing knowledge and experience of your development team.

You should only use the Azure Table API if you migrate from Azure Table Storage, as Core (SQL) offers far more features and flexibility.