ACOM = Ancestry.com
On-line = web-based
Member Tree = Member Tree
Program = software application
The website, Ancestry.com, (part of the Ancestry family of product) has a web-based genealogy software application. It's a basic gene program and is free to use. All that's required is registering for a free ACOM (Ancestry.com) account.
You have an ACOM account, it's required to post on the boards. If you're accessing the boards from rootsweb.com, you can use the same user name and password and log into the Ancestry.com site. Click on the "Learning Center" tab, there you'll find (somewhat lame) videos that walk you though the Member Tree program.
The on-line member tree program is not a "lite" version of Ancestry's desk-top apps, FTM and FTMM. It stands alone and doesn't "sync" with the Ancestry's desk-top apps. However, it's interface makes the import of data (a tree and the media attached to it) from the ACOM on-line program to FTMM smooth and intact. You can also "update" an existing same named tree/file on FTMM by importing the tree from the on-line member tree program. Tree data import should always flow in one direction only, otherwise you're begging for a mess.
I choose to use the ACOM on-line member tree program to make my data accessible to others. I do all my primary work on the ACOM on-line member tree program. Periodically, I import the ACOM tree to FTMM. I use the bells and whistles of FTMM (running reports, breaking of branches, etc) that the ACOM on-line program doesn't do.
Core changes, such as adding profiles, changes of profile data, deleting profiles, are all done on the on-line program, then I import the tree and "update" the corresponding FTMM tree file. I always work in the same direction ACOM to FTMM.
This keeps my data relatively sync'd and insures my on-line data is completely backed-up off-line and on my desk-top.
I've used (Mac native) Reunion for over 10 years, and know it cold. I prefer it's interface to FTMM, but times are changing. I want to have my data on-line, I also (pay) subscribe to access the (world) research databases on Ancestry.com. This all works for me, and my issues with FTMM have been minimal. I truthfully haven't had problems with it, and I've tinkered with it plenty. I started off small working with FTMM. I keep (small size, but tricked out) test databases on all programs to tinker around with, this enables me to mess around and learn the program without the fear I'll be doing some damage I can't recover from.
I work slowly and methodically, research takes time, and the data entry follows the research lead.