Without a qualifier, line usually means text line.
1. A text line (line of text) in a buffer is the longest sequence of contiguous characters, excluding the newline character (ASCII control character Control-J). In other words, the lines in a buffer are separated by newline characters.
Note that this definition of lines is independent of whether or not actual newline characters are present in the file that a buffer visits – what is important is what is present in the Buffer. In LongLines mode, for instance, newlines are inserted (as so-called “soft returns”) automatically at appropriate intervals for viewing and editing purposes; they are then removed when you save the buffer to a file. Those temporary newlines nevertheless partition the buffer into text lines, as described above.
Note also that, in some contexts, the newline character following a line is considered to be part of the line itself (its last character), but this is not really accurate terminology.
2. A window line or screen line is text that appears on the same line visually. A text line that is too long for the Window is sometimes wrapped to one or more additional window lines. The additional lines are known as continuation lines.