Under the Income-tax Law, there are four major assessments given below:
1) Assessment under section 143(1), i.e., Summary assessment without calling the assessee.
2) Assessment under section 143(3), i.e., Scrutiny assessment.
3) Assessment under section 144, i.e., Best judgment assessment.
4) Assessment under section 147, i.e., Income escaping assessment.

Assessment under section 143(1)
This is a preliminary assessment and is referred to as summary assessment without calling the assessee (i.e., taxpayer)

Assessment under section 143(3)
This is a detailed assessment and is referred to as scrutiny assessment. At this stage a detailed scrutiny of the return of income will be carried out. At this stage a scrutiny is carried out to confirm the correctness and genuineness of various claims, deductions, etc., made by the taxpayer in the return of income.

This is an assessment carried out as per the best judgment of the Assessing Officer on the basis of all relevant material he has gathered. This assessment is carried out in cases where the taxpayer fails to comply with the requirements specified in section 144.

This assessment is carried out if the Assessing Officer has reason to believe that any income chargeable to tax has escaped assessment for any assessment year