Prose can be best defined formally and can be defined in contrast to poetry and to other forms of writing (like certain lists or certain kinds of directions/instructions). 

"Prose - the ordinary form of spoken and written language whose unit is the sentence, rather than the line as it is in poetry. The term applies to all expressions in language that do not have a regular rhythmic pattern" (eNotes).

Prose is perhaps the most common mode of writing and uses the conventional forms of grammatically complete sentences. In prose, sentences are almost always presented on the page within a paragraph structure. 

Here is an example of a prose passage:

"Reading is the foundation of language arts. To be a competent writer, one must first be a good reader. Reading begins with learning that a language's alphabet is formed of individual letters that compose words, sentences, and paragraphs. Reading, however, is also being able to understand the larger meanings that exist within the printed word and often beyond it" (eNotes).

Notice that the writing in the passage above uses complete sentences structured into a paragraph format. 

If we take the same language and utilize a different form/format, we will see how prose differs from poetry. 

Reading is

the foundation

of language arts. 

Here, the form is poetic (although the language remains "prosaic"). The difference between prose form and poetic form is noticeable in the way the words are laid out on the page. Poetic form uses line-breaks and utilizes a stanza structure instead of a paragraph structure. 

If the writing uses complete sentences in paragraph form, it can be considered prose. This includes "prose poetry," which is something of a grey area between poetic language/expression and formal prose conventions (i.e., complete sentences and paragraph structure). 

If writing uses phrasing that does not amount to grammatically complete sentences then we will not consider it prose. For instance, a list of your favorite movies written with titles only in a bullet format is not prose if we adhere to the narrowest meaning of the term, though it may fall within the broadest limits of "prose." 

• Star Wars
• Karate Kid
• Frozen

Formally, this is simply a bulleted list and is neither poetry nor prose. 

Again, the defining features of prose are grammatically complete sentences and paragraph structure. 

There is more grey area in prose that we can discuss. Written dialog in a play can be considered prose, but applying the term prose to dialog in a play is mainly just a way to distinguish the writing from poetry or other forms. The form of writing in a play is "dramatic form," a term that is better suited to describe the mode of writing used in plays and skits. 

However, when we discuss written dialog in a novel or short story, we would conventionally define this form of dialog as prose. 

Also, if a piece of writing were to use a series of single sentence paragraphs, this too would be considered prose (as long as the sentences are complete and the format does not use line-breaks). 

As a rule of thumb, we might say that if a piece of writing is not poetry, then it is prose. However, there are clearly gradations within this large realm of language that can be refined to which currently have more precise descriptors than "prose" (e.g., dramatic form). 

Prose is the kind of language that most people use every day in stories, articles, and books.  Prose is straightforward and comes in two different forms:  narrative (such as stories) and expository (non-fiction writing). 

The characteristics of prose

1.  Prose does not conform to poetic measures.  In prose, the line is not treated as a single unit.  Whereas poetry is organized into couplets and stanzas, prose can be organized into paragraphs with no set or pre-determined line length. 

 2.  Prose does not have a specific rhythm or meter.   Although prose can often be very rhythmic, there are no set rules for metrical structure as in some forms of poetry.

3.  Prose does not rhyme or use a pattern of rhyme.