Types of Testing
There are 4 major categories of testing available out-of-box Drupal core. These tests focus on different aspects of code and functionality. Some part of the code could have tests on multiple types.
Basic tests without bootstrap/create Drupal site. Function/method level testing
Kernel test allows to do integration testing which helps to setup dependency injection container. Also allows to bootstrap/load required drupal elements like entity schema and module schemas.
Checks behaviour of an application as an user. This would allow to test end-to-end functionality.
When assert, labels, Button text, messages are not translated (i.e. passed in
t()) as these tests are running in single language instance. Check here for more details.