What is the difference between a journal whose name ends with "Letters" versus "Reviews"?
Letters and reviews are two different types of articles. Notably, letters are expected to be short, and reviews do not usually include new results.
Letter:

These papers communicate brief reports of data from original research that editors believe will be interesting to many researchers, and that will likely stimulate further research in the field. As they are relatively short the format is useful for scientists with results that are time sensitive (for example, those in highly competitive or quickly-changing disciplines). This format often has strict length limits, so some experimental details may not be published until the authors write a full Original Research manuscript. These papers are also sometimes called Brief communications.

Review:

Review Articles provide a comprehensive summary of research on a certain topic, and a perspective on the state of the field and where it is heading. They are often written by leaders in a particular discipline after invitation from the editors of a journal. Reviews are often widely read (for example, by researchers looking for a full introduction to a field) and highly cited. Reviews commonly cite approximately 100 primary research articles.

A journal titled &quot;Machine Learning Reviews&quot; is probably not going to consider letters, and vice versa - so do not submit there.