Microevolution is the change of gene frequency within a population. This can occur through genetic drift, which is just random change of gene frequency due to chance. The occurrence of a new mutation can cause microevolution, as the new gene resulting from the mutation mathematically displaces other genes. Natural selection can change gene frequencies in a specific population; this is particularly true if the population is a small one, or is isolated.
Macroevolution is a larger process, generally involving changes seen across an entire species rather than in just one population. Macroevolution results in speciation, in which a new species arises. Macroevolution is generally associated with long-term environmental changes, whereas a temporary change can cause microevolution. Macroevolution also usually takes much longer than microevolution.