Attestation is a legal acknowledgement of the authenticity of a document and verification that proper processes were followed. In short attestation is an official verification of something as true or authentic.
To prove the authenticity of our educational or non educational document before any foreign authorities we have to get relevant seal or signature (attestation) of concerned department or ministry of issuing country as well as embassies as per their requirement.

TYPES OF DOCUMENTS

Educational / Academic documents
All educational qualification of a person is coming under this category
Eg ; 10 th, 12th, Degree , PG etc..

Non educational documents

Any personal document other than academic documents such as birth certificate , marriage certificates etc are coming under this category 
Eg: Birth , marriage , death , single status certificate , one and the same certificate , income certificate , relationship certificate , personal power of attorney , affidavit  etc 

Commercial documents

Any document related to a company is coming under this category
Eg: certificate of incorporation , MOA , AOA , commercial power of attorney , invoices , commercial agreements , trade licenses , patents , certificate of origin , health certificates of products , export product list etc..

ATTESTATION PROCEDURES

1. HRD / GAD
    HRD stands for Human Resource Development as before 2005 this attestation was done by Human Resource Development Ministry of central government. On 2005 central government of India decentralized this attestation and directed all state governments to open a department to provide this attestation on the education documents issued from the respective states. Every states started a attestation department under their General Administration Department except Kerala. Kerala government started a wing to provide this attestation under Norka. Thus now the attestation is calling as GAD OR NORKA and also continuing the usage of HRD. 
   SDM/HOME
   SDM stands for Sub Divisional Magistrate. In Delhi home department coming under Central Government. Thus Delhi government authorized some Sub Divisional Magistrates to do attestation on behalf of home department in Delhi. So SDM attestation is equivalent to Home Department attestation in Delhi. 
   In other states a section of authorized officials under home department are there to do attestation on personal documents and this attestation is known as home attestation.
2. MEA / APOSTILLE- MINISTRY OF EXTERNAL AFFAIRS  ATTESTATION

 MEA : After HRD/SDM/HOME/GAD we have to get attestation from MEA ie. External (Foreign )  ministry of India have to be attest before producing the document in any Embassy