The most significant difference between medieval and renaissance art is that renaissance art paid more attention to the human body, and to detail. Both, however, focused mainly on religious themes, although not necessarily Christian. Renaissance artists are remembered because they brought about the changes that led to the art of today.
When looking at medieval art, such as "The Notary of Perugia Writing a Document" it is quite obvious that very little attention to detail is included. There is no depth to the painting, the writing on the parchment bares no resemblance to actual text, and everyone in the picture has the same face.
In contrast, when looking at a renaissance painting, like Christ the Redeemer by Titian quite a bit more attention is given to detail, even though the scene does not encompass nearly as much. It is possible to see shadowing in the painting, as well as to the behavior of fabrics.
There is also a good feel of depth with much attention paid not only to making a nice background, but separating it from the foreground as well.
There are many similarities in the two styles; they are, after all, separated by a short period in history. One similarity might be a choice of colors, as the most visually appealing color combinations had yet to be discovered. Another similarity would be the lack of understanding of how to accurately represent the human body, as this was considered sinful by the church; Leonardo di Vinci did began to change this with some of his works though.
The short period in time left similarities between the styles of art, but they were few, far between, and diminished fast. Renaissance artists put forth much greater effort into these works and it really shows in the quality, rather then the quantity produced.