You are actually talking about three different things;
Roots web is a public no fee site that has limited functions. Because it is public and free, a lot of folks post here who are just starting out and those who want to exchange information. You can only update either by individual or load a new Gedcom file when you have lots of changes.
Ancestry.com is a fee based program that provides a lots of bells and whistles, as well as a huge library of research records. You have a lot more options as to maintaing, updating, and sharing your data. Because it is fee based, access is limited and is not readily shared with non-subscribers. Sonic you have a lot of family members you want to share and update your research with, but are not Ancestry members, RootsWeb is the place they will need to go. You can export your Ancestry file to RootsWeb via a GedCom file transfer.
Finally, family Tree Maker. This is a stand alone program that resides on your personal computer. If you have an Ancestry subscription, then it links into Ancestery's data base and provides you with all it's resources to import into FTM.
Up until recently, your Ancestry and FTM data bases were not linked together and were therefore separate data bases. FTM just released FTM 2012 which does sync with Ancestry. Now whatever changes you make to one are made to then other.
In use FTM as my primary database. FTM has a lots of utilities to maintain and keep my records up to date, identify relationships, places, etc, that Ancestry don't have. Now with the sync features, I can keep Ancestry and all it's connection features current with FTM. You have the best of both world's. Between the two, you can effectively collaborate with others one your research and maintain clean and accurate records.
From the Ancestry and FTM data base, you can create a GedCom and update RootsWeb whenever you need to to keep the public side happy and up to date.
Hope this answers your query and gives some insight as to their purposes and functionality.
P.S. I can now access my synced Ancestry and FTM records via the iPad or iPhone app for when you are on the go or out doing research. Again, because of the sync capability, your Ancestry and FTM are kept up to date.