There are 23 exemptions from the general rights of access, split into two sections­ absolute and qualified exemptions.  
Qualified exemptions are those which will only apply where pressing public interest arguments can be made for withholding the information (the public interest in withholding it is greater than the public interest in releasing it).  
They are:
Section 22        Information intended for future publication
Section 24        National security (other than information supplied by or relating to named security                                organisations where the duty to consider disclosure in the public interest does not apply 
Section 26       Defence         
Section 27       International relations
Section 28       Relations within the United Kingdom
Section 29       The economy
Section 30       Investigations and proceedings conducted by public authorities
Section 31       Law enforcement
Section 33       Audit functions                      
Section 35       Formulation of government policy
Section 36       Prejudice to effective conduct of public affairs (except information held by the House of                            Commons or the House of Lords)
Section 37       Communications with Her Majesty, etc and honours
Section 38       Health and Safety
Section 39       Environmental information as this can be accessed through the Environmental                                            Information  Regulations
Section 40       Personal Information as there is already access to such information under the                                            DPA. Personal data about other people cannot be released if to do so would breach the                            DPA
Section 41       Legal professional privilege
Section 43       Commercial interests
Where a public authority considers that the public interest in withholding information requested outweighs the public interest in releasing it, the authority must inform the applicant of its reasons, unless to do so would mean releasing the exempt information.
Absolute exemptions are those where the public interest test does not apply and the public authority may withhold the information without considering any public interest arguments.
They are:
Section 21       Information accessible to applicant by other means
Section 23       Information supplied by, or relating to, bodies dealing with security matters
Section 32       Court records, etc
Section 34       Parliamentary privilege
Section 36       Prejudice to effective conduct of public affairs
Section 40       Personal information
Section 41       Information provided in confidence
Section 44       Prohibitions on disclosure where a disclosure is prohibited by an enactment or would                               constitute contempt of court.