The noncoding part of the human genome, which was previously considered nonfunctional or junk DNA, has been the subject of extensive research this decade. Nevertheless, long noncoding RNAs still represent one of the least investigated fields because of their complexity, multiplicity, and diversity. While some long noncoding RNAs have been characterized fairly well, the functions of many others remain poorly understood. Long noncoding RNAs play an essential role in the regulation of gene expression in all tissues and on all developmental stages. They are involved in a number of signaling pathways, and their aberrant functioning can be pathogenic. This review aims to summarize current state-of-the-art structures of these transcripts in this research field, their genomic localization, their functions, and underlying mechanisms. It also focuses on cancer-associated aberrations of long noncoding RNAs, as well as on prospects of their application in tumor diagnostics and therapy. Examples of decreasing the levels of oncogenic long noncoding RNAs via silencing with short interfering RNAs, antisense oligonucleotides, or low molecular-weight inhibitors are also described.