Every HTTP request includes an HTTP response. An HTTP response is a set of metadata and a response body, where the body can occasionally be zero bytes and thus nonexistent. An HTTP response will however always have response headers.

Response body

The response body will be passed to the write callback and the response headers to the header callback.
Virtually all libcurl-using applications need to set at least one of those callbacks instructing libcurl what to do with received headers and data.

Response meta-data

libcurl offers the curl_easy_getinfo() function that allows an application to query libcurl for information from the previously performed transfer.
Sometimes an application just want to know the size of the data. The size of a response as told by the server headers can be extracted with curl_easy_getinfo() like this:
curl_off_t size;
curl_easy_getinfo(curl, CURLINFO_CONTENT_LENGTH_DOWNLOAD_T, &size);
If you can wait until after the transfer is already done, which also is a more reliable way since not all URLs will provide the size up front (like for example for servers that generate content on demand) you can instead ask for the amount of downloaded data in the most recent transfer.
curl_off_t size;
curl_easy_getinfo(curl, CURLINFO_SIZE_DOWNLOAD_T, &size);

HTTP response code

Every HTTP response starts off with a single line that contains the HTTP response code. It is a three digit number that contains the server's idea of the status for the request. The numbers are detailed in the HTTP standard specifications but they are divided into ranges that work like this:
Transient code, a new one follows
Things are OK
The content is somewhere else
Failed because of a client problem
Failed because of a server problem
You can extract the response code after a transfer like this
long code;
curl_easy_getinfo(curl, CURLINFO_RESPONSE_CODE, &code);

About HTTP response code "errors"

While the response code numbers can include numbers (in the 4xx and 5xx ranges) which the server uses to signal that there was an error processing the request, it is important to realize that this will not cause libcurl to return an error.
When libcurl is asked to perform an HTTP transfer it will return an error if that HTTP transfer fails. However, getting an HTTP 404 or the like back is not a problem for libcurl. It is not an HTTP transfer error. A user might be writing a client for testing a server's HTTP responses.
If you insist on curl treating HTTP response codes from 400 and up as errors, libcurl offers the CURLOPT_FAILONERROR option that if set instructs curl to return CURLE_HTTP_RETURNED_ERROR in this case. It will then return error as soon as possible and not deliver the response body.