Question: I was trying to understand what is the difference between process and thread and I understood all the important points but I was not able to understand this line:

> 
> The typical difference is that threads (of the same process) run in a
>  shared memory space, while processes run in separate memory spaces.
> 
> 
> 

Here is the [reference](https://stackoverflow.com/a/200473/663148).

Can anyone help me understand what does memory space means here? And what does above line means in layman terms so that I can understand?

Answer: 
memory space is like a boundary. An isolation to not to overstep, overwrite each other hence processes cannot access each other's variable etc.,hence have to use IPC to communicate to each other. But threads can access global variables (typically use some kind of protection like mutex when reading writing ).

```
Assume ( T = Thread, P = Process, and V = variable ) 
P1 
global_var
T1 
  can set global_var = 2
T2 
  can read global_var
  and can ever reset global_var

P2
  It cannot read P1::global_var

```

Hope this helps!
There are couple of good answers already.

Some details :
Each process is given a virtual memory addresses with a range of 2 power word size ( 10 years ago it would have been 2 power 32 ( for 32 bit machines)). The process can uses these addresses *as if* it was physical memory. In the background the OS does a lot of swapping/paging/translation etc., from virtual to physical ( virtual address translation ). The OS makes sure when two processes reads or writes, the physical memories are not interleaved. But two processes can have the same addresses(virtual) when running but maps to different physical address in RAM/main memory.

Thread variables comes under stack and it is relative and is within the thread memory space/boundary hence an address in one thread cannot be reference in another but can access something outside of the thread's stack but within the process's address space.