whats the difference between amono hybrid cross a dihybrid cross? how are they similar?

A mono deals with only one trait while the dihybrid cross deals with two. The latter has more possible outcomes for the resulting offspring of the parents involved.

A monohybrid cross is when both parents are heterozygous for 1 trait (eye color for example; Bb x Bb). You will always get a 1:2:1 for the genotypes of the offspring.

A dihybrid cross is also when both parents are heterozygous, but for 2 traits (eye color and hair color; BbHh x BbHh). You will always get a 9:3:3:1 for the genotypes of the offspring.

Just to clarify the above comment....you will NOT always gets those ratios given....there are a whole range of ratios eg 1:3, 1:1 for monohybrids...but this is for complete dominance....there are a host of ratios depending on what type of gene you have eg complete dominance, incomplete dominance, co-dominance, multigenes, pleiotrophy!!!

Basically mono=1 and hybrid ( in this sense) is a cross.
You know its monohyborid as there is ONLY one letter used...eg A and a or T and t....THe punnet square will only have 4 squares and 4 possible options for offsrping ( so the offsrping numbers are usually out of 4 eg 3:1 , 1:2:1 OR 1:1 ( which is the same as 2:2, just simplified down)

Di=2 so this is a cross between 2 different genes, affecting 2 different traits like round and wrinkled pea and yellow/green colour ( typical example)...this is shown with 2 letters eg R and r ( for Round/wrinkled) and Y and y ( for yellow and green)
The punnet sqaure will ( usually)have 16 sqaures for possilbe offspring combinations eg 9:3:3:1.

They are both worked out the same way and are written in the same manner with the dominant allele ( if p[resent) written first, but the monohybrid will have a 2 combination offspring eg TT, Tt or tt (only options) while the dihybrid will have other possibilites eg RRYY, RrYY, rrYY, RRYy, RrYy, Rryy, rryy..I think thats them and written with 4 alleles ( 2 genes combined) and depending on the type of gene interaction ( complete dominance, incomplete dominance etc) the letter code may code  for different things.