Index And Data Pages

I am trying to understand how the data in sql server is stored and
also regarding fill factor and page splitting.

1) My first question what is the difference between Index pages and
Data pages.
and how are they different for clustered and non clustered indexes and
heap tables.

2) What is the relation between index and data pages.

2) BOL says that fill factor is used to define the amount of free
space on each page of index. I am confused here what does index pages
and data pages contain
for clustered/non clustered/heap tables.

3) Why does page splits occur and do they occur due to the lack of
space in index or data pages?


Thanks

skura