Texture nodes are nodes that actually hold and output an image (and as a consequence, they are resolution dependant). There are two kinds of texture nodes: the image and the buffer.
The image node outputs a single image from a file. Its unique parameter is the path of the image file.
The buffer node has a single input and two outputs. It renders its input into a buffer, and its outputs will read from that buffer. The first output emits the buffer in full resolution, and the second output will generate a given level of detail.
The buffer node has two parameters:
the size of the buffer
the detail level of the second output
Buffer nodes are generally used as input of shader nodes that sample their inputs multiple times (such as convolution nodes).