These are the Micronutrients that you supplement your tank with. While many of these elements are in your regular tap water, it is a necessity to add them for the health of your plants. You can buy them at many places. Here is a good link that could help you Introduction to Fertilizers for a Planted Tank
Traces are the nutrients that your plants use in trace amounts. The more commonly used trace supliments are flourish, CSM, and whatever TMG is called these days(TPN maybe?). There is also a product called flourish trace(different from flourish), it is not needed most of the time as tap water is about the same.