A purine will always pair with a pyrimidine. Examples of purines are adenine (pairs with thymine or uracil) and guanine (pairs with cytosine). Examples of pyrimidines are thymine (pairs with adenine), uracil (pairs with adenine), and cytosine (pairs with guanine).
There are equal parts of guanine and cytosine, and adenine and thymine, because they form base pairs in the DNA molecule. This is in accordance with the base-pairing rule, which states that in DNA, adenine always pairs with thymine, and guanine always pairs with cytosine.