what is privileged circumstances and professional communications? What is their importance in law of evidence?

It is well-established rule of law of evidence that no one can be compelled to disclose certain facts because they are privileged. Compelling a person to disclose such communications is against public policy. 

(1)    Communication during marriage 
No person who is or has been married shall be compelled to disclose any communication made to him during marriage by any person to whom he is or has been married; nor shall he be permitted to disclose any such communication, unless the person who made it, or his representative in interest, consents, except in suits between married persons, or proceedings in which one married person is prosecuted for any crime committed against the other.
It is clear that a husband and wife cannot be compelled to disclose the communication made to each other during their marriage without their consent the reason behind it is - 
1 Protect family peace 
2 Maintain faith between husband and wife during their marital status. 

Exception 
The privilege has two exceptions i.e situation when communication can be disclosed, are following 
(a) During civil suits between married persons 
(b) During such criminal proceedings in which one married person is prosecuted for a crime committed against the other.  

(2)    Evidence as to affairs of state 
It has been laid down under section 123 of the act, that no one shall be permitted to give any evidence derived from unpublished official records relating to any affairs of state, except with the permission of the officer at the head of the department concerned, who shall be given or withhold such permission as he thinks fit. 
Application of this section requires three contents - 
(a) Being an unpublished official record 
(b) Record must be related to affairs of the state 
(c) Disclosure of whom must be against the public interest. 

(3)    Official communication 
According to section 124, No public officer shall be compelled to disclose communications made to him in official confidence, when he considers that the public interests would suffer by the disclosure. 

(4)    Information as to the commission of offences 
Section 125 is intended for greater participation of the public in the matter of crime detection and offences against public revenue. It lays down that the police officer and the magistrate cannot be compelled to disclose the source of information as to the commission of the offence. The name and identity of informers are to be kept secret so as to encourage and assist police officers and magistrates in detecting offenders. They are also obliged by this section to maintain the secrecy of such information. It is public policy that source of information should not be divulged. The privilege contemplated in Section 125 is merely in respect of the source of the information.
 

(5)    Professional Communications 
Section 126 provides protection against disclosure of professional communication. According to it, “No barrister, attorney, pleader or vakil shall at any time be permitted unless with his client’s express consent, to disclose any communication made to him in the course and for the purpose of his employment as such barrister, pleader, attorney or vakil, by or on behalf of his client, or to state the contents or condition of any document with which he has become acquainted in the course and for the purpose of his professional employment, or to disclose any advice given by him to his client in the course and for the purpose of such employment

Provided that nothing in this section shall protect from disclosure—
(1) Any such communication made in furtherance of any illegal purpose 
(2) Any fact observed by any barrister, pleader, attorney or vakil, in the course of his employment as such, showing that any crime or fraud has been committed since the commencement of his employment. It is immaterial whether the attention of such barrister, pleader, attorney or vakil was or was not directed to such fact by or on behalf of his client. 
Illustrations
(a) A, a client, says to B, an attorney—“I have committed forgery, and I wish you to defend me”. As the defence of a man known to be guilty is not a criminal purpose, this communication is protected from disclosure.

(b) A, a client, says to B, an attorney—“I wish to obtain possession of the property by the use of a forged deed on which I request you to sue”. This communication, being made in furtherance of a criminal purpose, is not protected from disclosure.

(c) A, being charged with embezzlement, retains B, an attorney, to defend him. In the course of the proceedings, B observes that an entry has been made in A’s account-book, charging A with the sum said to have been embezzled, which entry was not in the book at the commencement of his employment. This being a fact observed by B in the course of his employment, showing that a fraud has been committed since the commencement of the proceedings, it is not protected from disclosure.