Careless is defined as a momentary lapse of attention or driving that falls below the standard of a careful and competent driver.
Dangerous is defined as a prolonged period of inattention or driving that falls far below the standard of a careful and competent driver.
All those judgments are subjective, which have to be made either by the magistrates or a jury if it's a crown court trial for dangerous.
Sentencing is clearer, you cannot be imprisoned for careless, but you can for dangerous,
You can be banned for careless, whether directly or by totting up if the mandatory six points takes you over the limit
A ban and retest is compulsory for dangerous, minimum 12 months.
The retest is defined as an extended test, so it is meant to be harder to pass than the original driving test.