Eicosahexaenoic acid (EPA) and docosahexaenoic acid (DHA) are long-chain polyunsaturated fatty acids (aka PUFAs). A mouthful to say, the two are "omega-3 fatty acids derived from marine sources," explains William Harris, Ph.D., FASN, FAHA, co-creator of the omega-3 index (a blood test that measures the amount of EPA and DHA in your red blood cell membranes).
EPA and DHA are produced by algae and consumed by a variety of fish, hence why fish oil and fish themselves are the richest sources of these fatty acids. They make up two of the three commonly talked about omega-3s—the third being alpha-linolenic acid (aka ALA), which is sourced from plants.
Our bodies can technically synthesize EPA and DHA from the parent omega-3 fatty acid ALA, through a series of reactions, including desaturation (i.e., the addition of a double bond between two carbon atoms) and elongation (i.e., the addition of two carbon atoms). That, of course, assumes you're consuming a whole lot of the starter fat (ALA) every day, and even then, the conversion is subpar.
That's because this synthesis process is affected by age, gender, and hormones—particularly the presence of estrogen. Research shows that only 8% of ALA is converted to EPA and 0 to 4% to DHA in healthy young cis-men, while 21% of ALA was converted to EPA and 9% to DHA in healthy young cis-women. Though the process is still not perfect, it appears women can convert ALA to be used in the body as EPA and DHA slightly more effectively than men.