We will be concerned with the spoken language.  We will consider the written language, its spelling (orthography) and grammar only as a reflection of the oral language.
        Our interest is in sounds, individual and combined, as the stuff of which meaning is constituted. Individual sounds can be represented by phonetic symbols. The standard notation of sounds is the International Phonetic Alphabet (IPA). One can indicate the sounds of comment allez-vous?  with recourse to the IPA: /kɔ mɑ̃ ta le vu/. The actual spelling might lead someone to mispronounce the question.  With the transcription in the IPA, there is little doubt about each of the actual sounds, provided that one is familiar with the IPA, of course.
        Some sounds are more important than others in any given language as a matter of communication. In the United States, for instance, some pronounce greasy /gri si/, whereas others say /gri zi/. What does it matter? Each pronunciation conveys the same notion without ambiguity or chance of faulty comprehension. Whether we hear /s/ or /z/, the meaning comes through. Consider, on the other hand, pen. Most Americans pronounce it as /pɛn/.  If pronounced as /pɪn/, it can be confused with pin. *
        The opposition between /s/ and /z/ matters little in the English-speaking world. The opposition between the sounds /ɛ/ and /ɪ/, on the other hand, is a significant one since it by itself alone may differentiate between two distinct meanings, especially in the northern United States. Thus /s/, /z/, /ɛ/, and /ɪ/ are all sounds in English. Of them two matter more since meaning may depend on them.
        French is made up of about thirty such significant sounds or phonemes. Phonemes are those sounds which constitute the smallest units of meaning in a given language. Notice that the smallest unit of meaning is not even a full word -- indeed, not even a mere prefix or suffix (syn- or -ed)! A phoneme may have associated with it similar though not identical sounds.  These associated sounds do not convey any meaning different from the phoneme to which it bears a resemblance. For instance, in English /s/ and /z/ are not phonemes. They do not always distinguish or imply different meanings, as we observed with two pronunciations of greasy. One can say that the sounds /s/ and /z/ are allophones or variants, if you like, of the same phoneme. Allophones can be thought of as variant manners of pronouncing the same phoneme. As for /ɛ/ and /ɪ/, we are dealing with two phonemes rather than allophones. *
        It is important to understand that sounds can accurately be described as phonemes or allophones only in reference to a specific language. There are no absolute phonemes and no absolute allophones that operate universally as such for all  languages.
        Each natural, historically evolved language possesses a deep structure of sounds played off against one another much like a succession of notes of various lengths and pitches within a musical composition. This structure is the language's sound system or phonology.
        In English, as we have seen, /s/ and /z/ are allophones; they are not phonemes, as meaning is not mediated by their justaposition. In French on the other hand, /s/ and /z/ are not allophones at all; they are phonemes. Which would you eat: /pwa sɔ̃/ or /pwa zɔ̃/?  There is a difference! Phonemes, and therefore allophones, are always relative to the sounds of a given language as a whole and make up a distinctive system of sounds conveying meaning. Phonemes may be consonant or vowels.
        Vowels are sounds which depend of the use of the vocal cords. Consonants are etymologically what goes 'with' (con-) sound; they are 'with' the sounds. We have sometimes the impression that all sounds are either vowels or consonants, but not both, as though there exists a dichotomy, such that all consonants are not vowels and all vowels are not consonants.  In point of fact, however, there are sounds which share the characteristics of both.  In French there are three semi-consonants or semi-vowels: /j/ /w/ and /ɥ/ as in fille, Louis and huit, respectively.
        The vocal cords are always turned on for the production of vowels; vowels are 'vocalized'. Consonants, on the other hand, are of two varieties: voiced consonants and unvoiced consonants. The distinction between voiced and unvoiced consonants is absolute; it is not relative to any particular language at all.
        Below in red are the unvoiced consonants used in French (and English):
                       /f/, /t/, /p/, /s/, /ʃ/ and /k/
                      /v/, /d/, /b/, /z/, /ʒ/ and /g/
Above in blue are the voiced consonants used in French (and English).
        A syllable is a single unit of rhythm composed, from a phonetic point of view, of at least one vowel and from zero to multiple consonants. For example, in the French word offrir, there are two (phonetic) syllables /ɔ/ and /RiR/. For donner, there are two syllables, as well: /dɔ/ and /ne/. For the phrase comment allez-vous, there are five syllables: /kɔ/ /mɑ̃/ /ta/ /le/ /vu/. Notice we are not concerned with the syllabification of writing or of grammar, but only with sound! Thus the third syllable is /ta/ combining a consonant and vowel of two distinct words.
        Most often in French (phonetic) syllables begin with a consonant and end in a vowel.  Although this predominant feature or characteristic of French phonology is by no means absolute, it is pervasive. It accounts by itself for matters of spelling and grammar that are actually determined by the sound system of French.  To give just one example, that is in part why it is mon étudiante, rather than ma étudiante. If c represents a consonant and v a vowel, then we have /mɔ̃ ne ty djɑ̃t/ or /cv cv cv ccvc/ so as not to be /ma e ty djɑ̃t/ /cv v cv ccvc/. French segments of speech tend to break into syllables of alternating consonants and vowels.  A typical segment of French speech can be represented often as /cv cv cv cv cv/.  Thus one implication is that word boundaries are effaced and suppressed -- something very unlike English or German.
        Syllables (phonetic, of course) are said to be either open or closed.  An open syllable is open if it ends in a (pronounced) vowel; it is closed if it ends in a (pronounced) consonant.  The word 'deux' is monosyllablic; it is an open syllable (/dø/) as it ends with the vowel /ø/. Sept /sɛt/, another monosyllabic word end in a pronounced consonant.  It is therefore a closed syllable.  What about août? Is this monosyllic composed of an open or a closed syllable?

* In the South, people often speak of an 'ink pen' probably because in the South /ɛ/ and /ɪ/ are not distinguished.