Dom mean by document object model. basically used for accessing each object in browser. Browser keeps all the tags in hierarchical node. Main Parent Element is Document Element. and then it has their childs. like html , head, body, and body having their child which we have written in Tag.
W3c have created one standard for each object in html elements. every elements (object) having some set of events , properties and their functions. basically it used when you are writing javascript code.
Make one thing sure that every node accessed in IE or FF their way of representing node is different. IN FF space will be treated as another Node. and in IE it will be doesn’t trach space as different node.
so incase you are accessing document.childElements[2].childElements[1] in IE might be different in FF. so be careful while doing programming.