Sheila and Larry, both gave excellent answers - Sheila is correct as to the symbolism of making the sign of the cross, and I agree with Larry that some Catholics utilize the "Sign of the Cross" more than others.
A couple additional things I would add are that
(1) The Sign of the Cross is also used by Orthodox Christians, though it is slightly different because Orthodox cross themselves right to left across their chest, while Catholics cross themselves left to right. This is because in Latin and other romance languages your hand would be over your heart when saying "spirit" if you crossed yourself left to right, and the opposite in Greek/Russian.
(2) I would definitely stress what Larry mentioned, that the sign of the Cross is in and of itself a prayer! It is also a way of "setting apart" our prayer time, it is kind of like a "hello" as we start our conversation of prayer!
(3) As for the "why" of many Catholics making the sign of the Cross when passing by a Church, it is to recognize the presence of the Eucharist/Christ in that place.