End cap cells are preplaced physical only cells that are required to meet certain design rules.
They are placed at the ends of the site rows, and are used in some technology for power distribution.
you shd add End cap cells to the design before any other standard cell is placed, but after hard macros have been placed in floorplan.
i.e, it is placed in a preplaced status, so future placement commands will not move them.........ok....
Added after 4 minutes:
Decap are nothing but Decoupling capacitance.
So adding these decoupling capacitance to a design can help maintain a stable voltage between pwr n gnd when signal nets switch.
this can reduce IR drop for power nets and limit bouncing on ground nets....ok
Added after 3 minutes:
Well taps r physical only filler cells that are required by some technology libraries to limit resistance between pwr or gnd connections to wells of the substrate.