Comments on use of DROPBOX & DROPSYNC

Hello
Not too sure this is in the right place as I've just moved over from W7 but here goes.

I have several devices, this W10 PC , a Samsung Tablet and a Samsung S5 phone. I want to be able to see an up to date display on any of the devices of a Budget app. I understand that I can set the update frequency and I have had it in use previously.

I've just upgraded to the Samsung S5 and thought I'd try and combine everything instead of having differing apps on the various devices.
So 1/ do I need both DROPBOX AND DROPSYNC. I seem to have a mixture of both on one and only sync on the other 2?
2/ will they update each other, so that if I update the phone that will update the other 2 devices and vice versa?
3/ heaven forbid I manage to delete the file on 1 device, but will that mean they will auto update the others to the same emptiness.
4/ dose this app keep several backups just in case I make a muck up as in 3/.

Thanks
EDIT>>> clearly I would benefit from a simple "How to" manual if there is such a thing, thanks again.

Stumbled my way through so all good now.......... I think.