I suspect that this is going to be rather short, but I think it will supply some useful information of interest. The question would seem to have a relatively simple answer, however there is some contention with regard to the use of terms here, and it is this which will be discussed in the following. I am going to be horribly biased with regard to this and focus on the use of the single sword in this discussion.
A ward is a position from which an attack or defense is launched. I think this is the best definition of what a ward is that I can find. A more in-depth definition of this would also include that this is a position which the combatant moves through from one action to another. What this means is that a set of actions might look like this: Ward - attack - ward - defense - ward - attack, and so forth. What is important is this is not a "guard".
A guard is a position in which one line is closed, thus the fencer is defended along that line due to being in that position, not requiring movement. The guard "Invitation to Sixte" protects the high outside line due to its position. If an attack is launched at the fencer along the high outside line the fencer does not have to move in order to defend himself. This is a guard, a ward does not do this in most instances.
The Low Ward, or Terza, is the favourite ward of many of the fencing masters and theorists due to its simple application in offense and defense. From this position it is convenient to parry to defend any line and also make attacks along any line that the combatant may want to. However an attack must be defended, and it must be defended by some action made by the combatant. This is not a guard, it is a ward. The defense must be made it is not already set by the position adopted.
While many may use the terms "ward" and "guard" together in order to describe the same thing, or even the same position, they are not the same. There is a clear difference as to what a true ward is and what a true guard is, and they are not the same. The swordsman should realise which he has adopted when he is facing his opponent, this can make a large difference. In a true guard he is defended along a single line due to his position. In a true ward it is a position from which an attack or defense is launched.