I've read multiple time sentences similar toFinally, for standard image classification bag-of-words  features based on SIFT descriptors have been found  critical for high performances. We first compute a  standard SIFT discriptor at regular grid points over  the whole image.Source: Multi-class image segmentation using Conditional Random Fieldsand Global Classification by Nils Plath, Marc Toussaint, Shinichi Nakajima. What is a descriptor? I thought  SIFT is an algorithm which operates on images and gives features (vectors in $\mathbb{R}^n$, where $n$ is fixed for a fixed size of images and parameters of the SIFT algorithm)?
What is the difference between 'features' and 'descriptors' in computer vision / machine learning?
machine learning;computer vision;features