mechanical downtilt : the whole antenna is tilted downwards.
electrical downtilt : the antenna is straight but the radio pattern is tilted, thanks to adjustment of antennas inner elements.
you need to downtilt an antenna because otherwise it is shooting straight, meaning half the beam is going over the horizon (propagating very far, creating interferences). And also, without a tilt, at the bottom of the antenna tower you might have a low signal strength (true if antenna is located high and there are not many buildings around (no reflections)).
Important parameters : half power beam width (from 60° to 260°), the vertical h.p.b.w. (from 15° to 3°), the gain (from 2dBi to 22dBi), the front-to-back ratio and of course, the electrical tilt..
for urban design, start with antennas with following characteristics : HPBW 65°, V-HPBW =7°, (gain should be about 18dBi), electrical downtilt = 6°.