Typically, Greek letter organizations are single-sex, initiatory organizations with membership considered active during the undergraduate years only, although a notable exception to this rule are historically black, Latino, Asian, and multicultural organizations, in which active membership continues, and into which members are often initiated long after the completion of their undergraduate degrees. Greek letter organizations may sometimes be considered mutual aid societies, providing academic and social activities. Some groups also maintain a chapter house, providing residential and dining facilities for members.

In modern usage, the term has become synonymous with the North American fraternity and sorority. The term fraternity, often colloquially shortened to "frat" (though use of such term may be derogatory in some contexts), typically refers to an all-male group, while the term "sorority" typically refers to an all-female group. However, some women's groups define themselves as fraternities for women or women's fraternities, such as Alpha Gamma Delta. Additionally, some groups that define themselves as "fraternities" may be mixed-sex, such as Alpha Phi Omega, Phi Sigma Pi, Alpha Delta Phi Society or Kappa Kappa Psi; the same is true of groups that define themselves as "sororities", such as Mu Phi Epsilon and Tau Beta Sigma. Due to the ambiguous nature of the terms "fraternity" and "sorority" with respect to gender, and due to the inaccuracy and potential sexist nature of the use "fraternity" to describe aforementioned organizations, it has become commonplace to use the synonym "Greek letter organization," since the vast majority of fraternities and sororities identify themselves using Greek letters. A recent example of this is the usage of the terms "(historically) Black Greek letter organizations" (BGLOs) and "Latino Greek letter organizations" (LGOs) within the literature. However, since most of those organizations that do not identify themselves using Greek letters are structured similarly to and share other several common characteristics with those that do identify themselves using Greek letters, all of these organizations are still considered to be "Greek letter organizations". All this said, the public at large and most members of fraternities and sororities still use the traditional terms ("fraternity" and "sorority"), to refer to all-male and all-female groups, respectively. Coeducational service fraternities and academic honors organizations (despite sharing a common history, as well as a common naming scheme, with modern fraternities and sororities) tend to be referred to more specifically. "Greek letter organization" tends to be used in "formal" contexts, but rarely in popular discourse.

The term social fraternity is used to differentiate four-year, undergraduate, and frequently residential groups from other organizations, many of which also have Greek-letter names, such as honor societies, academic societies, or service fraternities and sororities.

The names of North American fraternities and sororities generally consist of two or three Greek letters, often the initials of a Greek motto, which may be secret. For this reason, fraternities and sororities are referred to by the encompassing term "Greek letter organization" and described by the adjective "Greek," as seen in phrases such as "Greek community," "Greek system," "Greek life," or members as "Greeks." An individual fraternity or sorority is often called a "Greek house" or simply "house," terms which may be regarded as misleading since the usage of a phrase with "house" in it may be taken to refer to a chapter's physical property whereas many fraternities and sororities do not have a chapter house; alternatively, "chapter" and "organization" are used in these contexts, with the latter referring to the group as a collective entity, and the former referring to a specific division of such entity, though not all fraternities and sororities have multiple chapters.

The use of Greek letters started with Phi Beta Kappa (then a social fraternity and today an honor society) at the College of William & Mary – see History: Beginnings, below. Several groups, however, do not use Greek letters. Examples include Acacia, FarmHouse, and Triangle, as well as final clubs, eating clubs, secret societies at some Ivy League colleges, such as Skull and Bones at Yale and the military affiliated fraternity the National Society of Pershing Rifles.

Most Greek letter organizations are social organizations, presenting themselves as societies to help their members better themselves in a social setting.

A variety of Greek letter organizations are distinguished from social groups by their function. They can be specifically organized for service to the community, for professional advancement, or for scholastic achievement.

Certain organizations were established for specific religious or ethnic groups. Some social organizations are expressly Christian, such as Alpha Chi Rho (founded as Christian, presently non-exclusive). Jewish fraternities, such as Zeta Beta Tau (historically Jewish, presently non-sectarian), Alpha Epsilon Pi, and Sigma Alpha Mu, were established, in part, in response to restrictive clauses that existed in many social fraternities' laws barring Jewish membership, which were removed in the mid-20th century (Sanua 2003; Torbenson 2005). A controversy remains between the idea of creating supportive communities for distinct groups on the one hand and the intent to create non-discriminatory communities on the other.[citation needed]

There are also organizations with a cultural or multicultural emphasis. For example, Kappa Alpha Psi, an African American Fraternity, was established at Indiana University - Bloomington in 1911, the first Chinese fraternity, established at Cornell in 1916, and Sigma Iota, the first Hispanic fraternity, established at Louisiana State University in 1904 (Torbenson 2005). The latter later merged with other Hispanic fraternities and organizations around the nation to form Phi Iota Alpha, the oldest Latino fraternity in existence, in 1931. The Phi Sigma Alpha fraternity in Puerto Rico can also trace its roots back to Sigma Iota. There are now 20 Latino fraternities in the National Association of Latino Fraternal Organizations. A distinct set of black fraternities and sororities also exists, although black students are not barred from non-black organizations and there are black members of non-black organizations. Non African-American students are also not barred from predominately African American fraternities and sororities.

Organizations designed for particular class years do exist, but are usually categorized separately from other types of Greek letter organizations.[citation needed] While these were once common in older institutions in the Northeast, the only surviving underclass society is Theta Nu Epsilon, which is specifically for sophomores. Many senior class societies also survive, and they are often simply referred to as Secret Societies.