The tent pegs ensure fixing the tent in the ground. The pegs can be made of aluminum or iron; these can be round or with an angle of 90 degrees. Also, the pegs thickness determines their strength.
Aluminium pegs are preferred to those of iron, because iron rusts.
Thick pegs offer a better fit in the ground to the tent.
There are pegs that have a different shape than round: pegs of Y or V shapes. These pegs offer a better fit in the soil than the round ones.
V shape pegs:
Round pegs: 
The pegs can be purchased even after buying a tent. It is generally good to have some extra pegs in case you lose one or when one of the neighbors borrows a peg from you without asking. Although these are cheap accessories, pegs are extremely important when you have to deal with a torrential rain or a strong wind.