Chaos engineering is the practice of deliberately introducing failures to your system to understand how your system responds to different failures that can manifest to your users. For example, you may want to take down a random API or data service and see the side effect on your users’ applications. If the side effects are impactful and if you have Service Level Agreements (SLA) in place, then this can have a detrimental impact on your organisation.

In its simplest definition, contract testing is used to test that two systems have a shared understanding of expectations. For example, when a web application sends a GET request to a data service, the web application has expectations regarding how the data service should respond. Contract testing captures the interactions between these two systems, which are then stored in a contract that both systems must adhere to.

User stories are short and simple descriptions of a feature or functionality that are told from your user's perspective

In software testing, heuristics are like mental models or shortcuts that we as testers use to guide our work. I like to think of heuristics as strategies that help me generate new ideas on what to test and how to test it especially when I run out of ideas myself. Heuristics are particularly useful when it comes to doing exploratory testing because they give us a guideline on what to explore next. 

A test idea is more about the general thought of what to test in a software. They are often generic and it involves looking at the wider picture.