There are 2 questions here:

A. split functionality for different users.
You can use the exact same FE for all users without tweeking at
install:
1. Create a staff table with user IDs, names passwords etc. and decide
on different fields which describe what category of user they are and
maybe some fields representing yes/no things they can/ can't do.
2. On login launch a form which takes a user name and checks the
password
3. In a temporary front end table if the login is correct put in the
user ID, login time, whatever other info you want to record.
4. Dynamically in the code of the database, you can select different
forms depending on the rights associated with the user ID - and within
the forms even allow/ prevent options being used. For example:

function ctrlButton_click()
Dim User as Integer
User = dlookup("[UserID]","[tblUserTmp]")
if dlookup("[CanAddRecords]","[tblStaff]","[UserID] = " & User) = -1
then
 docmd.openform "frmReadWriteForm"
Else
 docmd.openform "frmReadOnlyForm
end if
end function

or in the form load function of a form you could put something similar:

Dim User as Integer
User = dlookup("[UserID]","[tblUserTmp]")
if dlookup("[CanViewFinance]","[tblStaff]","[UserID] = " & User) = -1
then
 me.txtFinance.visible = true
Else
 me.txtFinance.visible = false
endif

This isnt exactly iron clad security or anything close to it, but it is
useful to seperate user functionality. If your feeling brave you can
tie it in to the access user groups for a bit of real (ish) security.

B. How does replication work?
There is a lot written on this topic by people who know more than me -
(check out microcoft.public.access.replication) but a basic overview is
this:

When you create the backend you replicate it using the wizard and place
the "design master" of it on the server. Place a replica backend then
on each pc. when a pc is networked, the information on the design
master and the local backend can be synchronised so that changes are
updated across the two and they then hold the same data after
synchronisation. when on the road, users use the local backend and when
they are at the office they synchronise the data and use the network
backend.

There are a lot of issues about conflicting updates, deletes, and
possible corruptions and their resolutions. It's not impossible - but
it's not easy either and its worth researching before you jump into it.

Hope this helps