Merriam-Webster's Dictionary proposes two definitions for slang. The first definition refers to a type of speech that is grammatically incorrect and used by a certain locale. For example, in Arkansas, a person's southern accent might lead him to say, "I'm fixin' to go to the store." This means he is about to go to the store, and "fixing" is a slang term for "about to."
The second definition of slang builds on this idea and refers to a type of speech that uses misspellings, changed words or excessive figures of speech. For example, a girl might say this to refer to a man, "That guy is hot." This means she finds him attractive, which she emphasizes by using the word "hot."
Lingo refers to speech that is specialized for a certain profession, field of interest or foreigners. For example, lingo is often used by police officers to avoid revealing information to the public. A police officer may say, "We've got a one-eight-seven on 6th Street." "One-eight-seven" is the code for homicide, and it corresponds to the booking number for homicides, which is 187. The language of foreigners may also be called lingo if it seems incomprehensible or difficult to understand.
Lingo and slang differ in terms of their correctness grammatically. Slang and lingo may both be incomprehensible to those who do not understand the patterns of speech, but slang uses idioms and nonstandard words to express its meaning. Lingo primarily uses acronyms and jargon that is associated with a profession. In this sense, the lingo used by professionals or foreigners is a planned series of expressions that serve a purpose. Slang is a more organic form of expression that results from misspellings and word equivocation over time.
Slang often populates pop culture, whereas lingo finds itself in more professional settings. For example, slang may be heard in a rap music video, with phrases such as "we on," "on point" or "fresh to death." Lingo, such as "PT," "CT" or "HIV" may be heard in a doctor's office to refer to procedures or illnesses. In this sense, the word "lingo" is a lot like the word "jargon" because both refer to an unintelligible and professional form of communication. Jargon, however, often has the stigma of being a pretentious and unnecessary form of communication that seeks to use flowery language to confuse listeners.