Bug:--  Bug is identified by the tester.
Deffect:-- Whenever the project is received for the 
analisys phase ,may be some requirement miss to get or 
understand most of the time Defect itself come with the 
project (when it comes).
Issue :-- Client site error most of the time.
Error :-- When anything is happend wrong in the project 
from the development side ie. called as the error ,most of 
the time this is knowing by the developer.

Bug : a fault or defect in a system or machine 

Defect : an imperfection in a device or machine;

Issue : An issue is a major problem that will impede the 
progress of the project and cannot be resolved by the 
project manager and project team without outside help

Error :
Error is the deviation of a measurement, observation, or 
calculation from the truth

Bug : Logical deviation- it will effect on the system but 
not known until we found the bug. ( Clock completes one 
cycle in 11 hrs instead of 12hrs)

Deffect: Mismatch with expected results. (Ex: Clock 
rotating in anticlock direction)

Error: System will stop functioning. Reason unhadled 
exception or syntax or special chars. 

Issue : Descriptive Functional points like Change request.

Error: 
•This is terminology of developer
•Syntax error, logical error, run time error, compilation 
error etc..
Bug: 
•This is terminology of tester
•Mismatch between the Expected result & Actual result.
•Anything that causes customer dissatisfaction.
Defect: 
•Deviation (Mismatch) from the client requirement / 
Specification
•Eg: Add func  2+2 = 4 (correct); 2+2 = 0 (defect)
Issue:
client having fault in the project or product.