Transaction signature algorithm
The ECDSA digital signature scheme returns two values. To be specific, the X and Y values computed on the elliptic curve are returned.

In Bitcoin the signture is DER encoded, which is represented as a string containing the X and Y values and also some header data. But both X and Y can easily be extracted from it when reading the string from left to right. 

In fact the Bitcoin protocol uses more information than only those concerning the X and Y values. For example, the size of the DER encoded signature is specified, but also a so called signature type is used to define which part of the transaction the signature was computed for.

Take a look here under the section "Signing the transaction" to see exactly how it works. In the table you will see how a computed signature is used in a scriptSig.

The details regarding the DER encoding was discussed here.