Watch a youtube video of someone building a new pc (linustechtips etc)
As they install each part do some research on that part (what it is, what it does.. etc)
Look at the generations of processors (AMD- (phenom, fx etc) Intel- (pentium, i3,i5,i7,xeon)
Generations of Graphics cards (AMD- (HD series, R7 series, R9 series) Intel- ( GTX 5XX, GTX 6XX, GTX 7XX, GTX 9XX)
Different Ram (DDR1, DDR2, DDR3, DDR4) + timings 800mhz-2400mhz + dual channel, quad channel
Power Supplys ( 80+ ratings, power output thats neccessary, amps on 12v rail, single rail vs dual rail)
Harddrives ( size (1tb,2tb etc), speed (5400rpm, 7200rpm), (colour varients green,blue,black)
Solid State Drives (SSD) Difference between HDD and SSD, speed etc
Sound Cards...