We've gone over this from time to time, but what the heck, this will make it easier for people to find the answer:
There is a superficial similarity between the rood screen and the iconostasis, but but the screen doesn't have any liturgical function. If you take the classic tripartite medieval church, the screen separates the choir and the nave; the iconostasis, however, divides the church at what it the west would be the line between the sanctuary and the choir, where in a classical Anglican church there would be the altar rail. The screen divides the "east" end of the church into a separate subchapel for use in office services by the clerics and religious. This particularly is evident in a monastic church, where the brothers/sisters occupy the choir; the screen in this case marks the division between the enclosed monastery and the outer world. The screen also allowed the nave to be used a secular (or at least non-worship) meeting hall.
When the whole church was in use for a single worship (e.g. Sunday mass) the screen was largely useless, if not an actual impediment. It has never had any liturgical functions, though in medieval churches it might have altars set up against it for votive masses. Renaissance and Baroque churches omitted it, as did Anglican Georgian churches. These churches generally adopted a plan in which the choir (now composed of singers rather than clerics) was in a gallery in the back, and the pulpit (for Anglicans) or altar (for Catholics) was emphasized. When the gothic revival came along, well, gotrhic churches had screens, so by God, neogothic churches needed to have screens too. The problem was that after 250 years, nobody now knew what they were for, and indeed, the church culture and practice had changed so that the screen was of no use at all. So some people figured this out, and constructed buildings which kept the tripartite arrangement but omitted the screen; and some people kept the screen but fused it with the altar rail (by this point a universal fixture); and some particularly rock-headed types put it in its tradition location. To take a particularly perverse example, Frohman not only put a big screen in the national cathedral, but canted the choir slightly, so there is no place you can see the high altar from except the choir and one end of the west end gallery.
The rood itself (which wasn't necessarily part of the screen) had devotional functions, particularly on Good Friday. The regular eucharistic and office liturgies made no reference to it, however.
