As far as I know, the symbols [s] and [z] typically refer to consonants with the same place of articulation, in terms of tongue and lip position (that exact place is different in different languages).
They are also both fricatives.
Differences in voicing often go with differences in pitch contours
It is also supposed to be common for there to be difference in the pitch transitions betwen voiced and voiceless consonants and the following vowels (as mentioned in this Language Log article by Mark Liberman: Consonant effects on F0 of following vowels, although it doesn't look at fricatives). I'm not sure how much of that (if any) is an automatic consequence of the difference in voice, but it seems that in some languages different pitches can become phonologized in syllables that originally had voiced or voiceless consonants.
Differences in voicing often go with differences in duration
I've read that in many languages, [s] and [z] tend to have a difference of duration, with [s] being longer than [z]. If I remember correctly, this is the case for the /s/ and /z/ of English, German, and Navajo (in the latter two languages, the phonetic voicing of /z/ may be weak or even absent). But either sound can be prolonged, so I don't think that a difference in duration is generally essential for the contrast.
As Araucaria mentions, in certain contexts, the duration of the preceding vowel is related to the /s/ vs. /z/ contrast in English, and I think I've read that this is also detectable in German, but I don't think this is a universal phenomenon, and I'm not sure how common it is outside of Germanic languages.
The meaning of "fortis" and "lenis"
"Fortis" is a somewhat common term in literature, but I don't know what it means when talking about the phonetic qualities of sounds.* Unlike Greg Lee, I can't directly hear or feel a difference in the amount of force when I pronounce [s] and [z]--the idea of "forceful articulation" feels more like a metaphor to me, like calling palatalized consonants "soft" or velarized consonants "hard".
Since it's come up in both Greg Lee's and Araucaria's answers, I thought I'd do more research on what "fortis" means. The preceding paragraph, which is what I wrote originally in this answer, is based on my inability to perceive this quality through personal introspection, as well as some statements in the Wikipedia article that suggest that it is difficult or controversial to identify which measurable correlates of articulatory force (aside from voicing) are connected to phonological contrasts between consonants.
I found paper with some discussion that helped me a bit. "The Phonetics of Fortis and Lenis Consonants in Itunyoso Trique", by Christian T. DiCanio, has a section titled "Defining 'strength'" (1.2, p. 244-245). DiCanio says
Malécot (1966) attempted to provide some phonetic grounding for differences in articulatory energy between consonants. He concluded that the “intra-buccal air pressure impulse” was the primary correlate of force of articulation, where stops [p] and [t] were produced with greater intra-buccal air pressure than stops [b] and [d]. [Footnote 3: Yet Malécot did not consider that differences in voicing would have significantly contributed to the lower oral pressure and amplitude values that he observed.]
(p. 244)
There are three phonetic correlates which unambiguously reflect a stronger
primary articulation. First, one expects stronger articulations to involve greater constriction between two articulators (Fougeron and Keating 1997, Keating et al. 2000, and Lavoie 2001). The only articulation that can be responsible for this is greater muscular tension between the articulators. Second, one expects stops to have louder bursts if they are produced with greater muscular tension. Articulators with more muscular tension will close and release more quickly, causing burst intensity to increase (Debrock 1977 and Kohler 1984). Related to this is the fact that increased stiffness among articulators will result in more abrupt formant transitions into and out of a consonant and faster falling and rising intensity contours (Debrock 1977). Thus, one usually expects consonants with increased articulatory strength to be abruptly timed relative to the surrounding segments. While differences in the constriction degree of consonants are usually measured using articulatory measures (e.g., electropalatography [Lavoie 2001]), both burst amplitude and formant and intensity transitions can be measured acoustically
(p. 245)
I don't know whether differences in any of these articulatory qualities have been found for a [s]/[z] pair in some language. DiCanio says burst intensity is relevant specifically for plosives, but [s] and [z] are fricatives.
I think lower constriction is a fairly common characteristic of the voiced counterparts to non-sibilant voiceless fricatives. E.g., in many languages, voiced "fricatives" like [β], [v], [ð], [ʝ], [ɣ], [ʁ] fairly often can be realized more like approximants than their voiceless counterparts [ɸ], [f], [θ], [ç], [x], [χ], and few languages have a contrast between voiced approximants and voiced fricatives at the same place of articulation: e.g. [v] and [ʋ] or [ɣ] and [ɰ]. But I'm not sure whether this is true of sibilant fricatives: I can't tell whether a less constricted, more approximant-y realization of [z] is possible. Diachronically, [z] can be subject to change into a rhotic approximant (a sound change called rhotacism), but the result of that change is no longer a sibilant consonant.
