Myc (c-Myc) is a transcription factor.
It is an oncogene.
Myc gene encodes for a transcription factor that is believed to regulate expression of 15% of all genes  through binding on Enhancer Box sequences (E-boxes) and recruiting histone acetyltransferases (HATs). This means that in addition to its role as a classical transcription factor, Myc also functions to regulate global chromatin structure by regulating histone acetylation both in gene-rich regions and at sites far from any known gene.
A mutated version of Myc is found in many cancers which causes Myc to be persistently expressed. This leads to the unregulated expression of many genes some of which are involved in cell proliferation and results in the formation of cancer. A common translocation which involves Myc is t(8:14) is involved in the development of Burkitt's Lymphoma. A recent study demonstrated that temporary inhibition of Myc selectively kills mouse lung cancer cells, making it a potential cancer drug target.