Question: What is tahāra?

Answer: Tahara is ritual purity. Tahāra is necessary for ritual practices like prayer.

Question: What is najāsa?

Answer: Najasa is ritual impurity. Excrement, blood or semen are considered najis or ritually impure in Islam.

Question: What is tayammum?

Answer: Tayammum is a form of ritual purification with the same effects of ghusl. It is performed when one does not, for example, have water.

Question: If I perform ghusl, do I have to perform wudū as well?

Answer: Generally no. There are cases which it may be needed, but this is subject to disagreement among Muslim scholars.

Question: Does najāsa or janāba mean dirty?

Answer: No, it means ritual impurity. Something may be ritually impure, but be considered clean in the conventional sense. The same applies to tahāra.