WAJIB: obligatory, necessary, incumbent. An act which must be performed. A person will be rewarded for performing it and punished for neglecting it, e.g., the daily prayers.
IHTIYAT WAJIB: precationarily obligatory. Its significance is the same as that of wajib with the difference that wherever a mujtahid says that "it is precautionarily obligatory," his followers have the option of leaving his taqlid (following) in this particular problem and following the fatwa of the second best mujtahid provided the second mujtahid has a different opinion.
HARAM: forbidden, prohibited. It is necessary to abstain from the acts which are harm. If someone performs a harm act, he will be punished either by the Islamic court or in the hereafter or both. For example, stealing, eating pork.
SUNNAT or MUSTAHAB: recommended, desire-able, better. It refers to the acts which are recommended but not wajib. If one neglects them, he will not be punished; however, if one performs them, he will be rewarded. For example, washing the hands before wudu.
MAKRUH: reprehensible, disliked, discouraged. It is used for the acts which are disliked but not harm. If someone does a makruh act, he will not be punished for it; however, if he refrains from it, then he will be rewarded. For example, eating before ghusl janabat.
JA'IZ, HALAL, MUBAH: permitted, allowed, lawful, legal. The acts or things which are permitted and lawful. There is no reward for performing it nor any punishment for neglecting it. Mubah is exclusively used for lawful things, not for permitted actions. An example, drinking tea.
MUJTAHID: a religious scholar who is an expert of Islamic laws, the shari'ah. Usually, it is used for the high ranking mujtahids whose decrees are followed by the people.
MARJA: The high ranking mujtahid is who is followed by the people. Literally, it means the point of reference. The high ranking mujtahids are called marja` because they are the point of reference for the people in the shari`ah matters.
SHARI`AH or SHARI`AT: Literally means, the way. In Islamic terminology, it means the laws of Islam.