Beginner:user ID's and group ID's

Hello Gurus,
I just started Unix, i am neither a student nor a proffessional.

i just completed my masters degree and looking for a career in Unix/Linux.
so i will be preparing for myself with the help of different forum's guru's.

i just completed the os basics and going through baiscs of unix.

according to the knowledge i got from books a user ID is used for each user of the Unix as unix is a multiuser operating system.

but what for this effective UID?

group id for a group of people working under the same project.

whats this effective group id.

i have read the effective user id , but didnt understand it completely and its relation with setuserid.

please help me.

i am using richard stevens

thanks

google .... and following seems useful.

Real and Effective IDs

thank you very much geek ,

your post was so help full.

i have gone through that and i understood well about the effective Ids.

but i have a confusion regarding setuid().

please correct following:
setuid is used by the process to gain access to the other users resources
by setting effective user id of the process to resource owner's id.
but ,

1. how the process determines which resources it can access, which uids it can set.

2. can a process set the euid of root to any other process.
   getting the root privilliges to the normal process.

Thanks,

I hope the following experimentation can guide you better.,

1. C program for showing the EUID usage.

$ cat t.c
#include <stdio.h>

int main()  
{
    printf ("My UID: %d\n",getuid());
    printf ("My EUID: %d\n",geteuid());
}

2. Compile it

$ cc t.c

3. Execute it

$ ./a.out 
My UID: 1000
My EUID: 1000

4. Change the owner to root, and set userid.

$ sudo chown root:root a.out
$ sudo chmod u+s a.out 
$ ls -l a.out 
-rwsr-xr-x 1 root root 8373 2009-11-09 18:52 a.out

5. Execute the program as normal user, so user id is 1000, and euid is 0.

$ ./a.out 
My UID: 1000
My EUID: 0