Question: What is the difference between wajib and fard?

Answer: Fard is a subcategory of wajib, it is a Quranic obligation, i.e. an obligation explicitly found in the Quran. 

Question: What is mustahab?

Answer: Mustahab is a recommended act in Islam which one gains a lot of reward for doing, but no sin for not doing it. 

Question: What is Makruh?

Answer: Makruh is a hated act in Islam, but one gains no sin in doing it, but gains rewards from God for desisting from it. 

Question: Can something mubah become wajib?

Answer: Yes, regularly drinking water is mubāh. However, if regularly drinking water means saving your life because of some condition you have, mubāh becomes wājib. 

Question: How many categories of law do we have in Islamic practice?

Answer: Five. They are wājib, mustahab, mubāh, makrūh and haram.