The major difference between threads and processes is 
1.Threads share the address space of the process that 
created it; processes have their own address.

2.Threads have direct access to the data segment of its 
process; processes have their own copy of the data segment 
of the parent process. 

3.Threads can directly communicate with other threads of 
its process; processes must use interprocess communication 
to communicate with sibling processes. 

4.Threads have almost no overhead; processes have 
considerable overhead.

5.New threads are easily created; new processes require 
duplication of the parent process.

6.Threads can exercise considerable control over threads of 
the same process; processes can only exercise control over 
child processes. 

7.Changes to the main thread (cancellation, priority 
change, etc.) may affect the behavior of the other threads 
of the process; changes to the parent process does not 
affect child processes. 

Process is a program in execution where as thread is a 
seperate path of execution in a program.

process is a execution of a program and program contain set 
of instructions but thread is a single sequence stream 
within the process.thread is sometime called lightweight 
process. single thread alows a os to perform singler task 
ata time similarities between process and threads are: 
1)share cpu. 
2)sequential execution 
3)create child 
4)if one thread is blocked then the next will be start to 
run like process. 
dissimilarities: 
1)threads are not independent like process. 
2)all threads can access every address in the task unlike 
process. 
3)threads are design to assist onr another and process 
might or not might be assisted on one another.