[rbayer]

Correct me if I'm wrong, but the notation is just standard set notation. For example, {x in R | x > 2} would be read as "all x beloning to the set of real numbers such that x is greater than 2."

Similarly, {P | d_T(P, A) = d_T(P, B)} would be the set off all points equidistant from two fixed points (A and B, in this case). Similarly, the set {P | dT(P, A) + dT(P, B) = dT(A, B)} would be the set of all points that are colinear with fixed points A and B (ie the sum of the distances from a point to each of the fixed points is the same as the total distance between the two fixed points).

Further examples:

{P | d_T(P, A) = r} is the set of all points of fixed distance from A (ie a circle of radius r centered at A.)

{P | d_T(P, A) + d_T(P, B) = c} is an elipse


As for taxicab geometry, if this kind of thing interests you, there are entire branches of mathematics devoted to the study of non-Euclidean gemetry. I took half a semester of this stuff last year, so if you have any questions, please post and I'll try to answer. Also, if you want to do more research on your own, the formal mathematical name for this kind of thing is a Metric Space.