The Four Seals are:

Schools of Buddhism based on the Buddha's teachings accept the Four Seals of Dharma as the distinction between true Buddhism and "sorta  looks like Buddhism." Further, a teaching that contradicts any of the Four Seals is not a true Buddhist teaching.

1. All compounded things are impermanent.
All things are impermanent even a mountain -- certainly a mountain may remain a mountain for 10,000 years. But even 10,000 years is not "always." The fact is that the world around us, which seems solid and fixed, is in a state of perpetual flux.

2. All stained emotions are painful.
His Holiness the Dalai Lama translated this seal "all contaminated phenomena are of the nature of suffering." The word "stained" or "contaminated" refers to actions, emotions and thoughts conditioned by selfish attachment, or by hate, greed and ignorance.

3. All phenomena are empty.
Another way to say this is that nothing has intrinsic or inherent existence, including ourselves. This relates to the teaching of anatman, also called anatta.

4. Nirvana is peace.