[ccp4bb] on space group

[Smith Liu]

Dear All,
 
Alhough there are on-line explainations on the space group, I found it was 
difficult to fully understand. Here we take P 1 21 1 as an exmaple, will you 
please explain to me with easy language what each number indicates？Or do we 
have a on-line server which can demonstrate the meaning of each number? How can 
we get out crystal arrangement with repeating the unit cell in the format of 
space group?
 
Smith