explain transistor as a switch

 Transistor works as a switch in cut-off & saturation mode..

See, When Base-Emitter(BE) are connected in frwd bias & Collector Emitter are in revrs bias..4 Common Emitter mode

1st we keep fixd value across Vce constant and vary Vbe which indirectly varies Ib{Current thru base} ....When the value of Vbe reaches abt 0.7 for silicon or 0.2 for Ge Thers a drastic incrse in Ib...& There is a flow of low  current thru collector i.e. Ic.....So we keep Ib constant now & Thus earlier switch was in off mode when Ic was 0...After some time Ic saturates & Now switch is in on mode....

Hope u understood ...ITs not so difficult...Refer these diagrams