by Jackson D.

SQS

SQS (Simple Queue Service)

Places messages into a queue. Applications pull queue using AWS SDK

Can ensure messages are delivered at least once
Can ensure only one message is sent

Usefull for delayed tasks like queueing up emails.

You can subscribe SQS queues to an SNS topic to send SQS queues to multiple destinations.

SNS

SNS (Simple Notification Service)

Send notifications to subscribers of topics via multiple protocol. eg, HTTP, Email, SQS, SMS.

Can retry sending in case of failure for HTTPS

Great for webhooks, simple internal emails, and triggering lambda functions.

Lambda

AWS Lambda is a serverless, event-driven compute service that lets you run code for virtually any type of application or backend service without provisioning or managing servers.

Can be triggered from over 200 AWS services
Only pay for what you use

Amazon RDS

Amazon Relational Database Service (Amazon RDS) is a collection of managed services that makes it simple to set up, operate, and scale databases in the cloud.

There are seven popular engines,

Amazon Aurora with MySQL compatibility
Amazon Aurora with PostgreSQL compatibility
MySQL
MariaDB
PostgreSQL
Oracle
SQL Server

You can also deploy on-premises with Amazon RDS on AWS Outposts.

You can achieve high availability with Amazon RDS Multi-AZ deployments.

DynamoDB

DynamoDB is a fully managed, serverless, key-value NoSQL database service.

DynamoDB stores 3 copies of data on SSD drives across 3 AZs in a region.

DynamoDB has two capacity modes, Provisioned and On-Demand. You can switch between these modes once every 24 hours.

Provisioned is suited for predictable or steady state workloads
You should enable Auto Scaling with Provisioned capacity mode. In this mode, you set a floor and ceiling for the capacity you wish the table to support. DynamoDB will automatically add and remove capacity to between these values on your behalf and throttle calls that go above the ceiling for too long.
On-Demand is suited for new or unpredictable workloads
On-Demand Capacity is pay per request. So you pay only for what you use.