[Zope] What use is BTrees ???

I am unshure on how BTrees works, but I suspect that they might be usefull in a project I am doing, so I thought I might ask here. I have an object which hase a dict as an attribute, in which I store other objects. I know this is a relative expensive way to do it as everytime an object is added to the dict the full object will be stored in the ZODB. As far as I understand, the BTree is like a dict. So do I understand it right that if I use a BTree as a attribute instead of a dict, it will only store the elements that are added/changed in the ZODB but otherwise act as a dict? I use integers as keys in the dict regards Max M