For my film I was thinking of... getting a composter to do it...
Compost is organic matter that has been decomposed; a composter does that job. It is not to be confused with a composer, someone who composes music. (I have heard scores that were good for nothing but compost...

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I'll consolidate and comment.
Synthesizers are electronic devices that uses various processes to create sounds. The original analog synthesizers used VCOs (Voltage Controlled Oscillators) to create the basis of the sound. The VCO could create sine, saw and square waves. The infamous MiniMoog had three (3) VCOs. The sound resulting from the VCO was run through a VCF (Voltage Controlled Filter); a VCF allows its cutoff frequency and Q factor to be continuously varied; it usually gives a lowpass response, but may also be switchable to allow highpass, bandpass or even notch responses. After the VCF comes the VCA (Voltage Controlled Amplifier); this controls the durations of the Attack, Decay, Sustain and Release (ADSR) of the sound. Some synthesizers also have ADSR for the filter as well as the amplitude. You can also modulate sounds (add vibrato of various types and depths), bend the pitch and use dozens of other real time controllers.
Modern synths recreate this scheme digitally, in most cases with the exception of the VCO. The old Yamaha DX7 and related synths used FM (Frequency Modulation) to produce complex waveforms. Most current synths use some form of PCM (Pulse Code Modulation), Granular Synthesis or Physical Modeling.
Digital sampling is a different process altogether; you record an actual sound (or dozens of them) and map the sound(s) to a keyboard. The sounds can still be modified with "traditional" Frequency, ADSR and other controls.
There are also orchestration libraries that attempt to provide the infinite number of articulations for the various instruments in solo and ensemble format. Besides individual notes the libraries may also include instrument phrases.
The imitation of real instruments and their subtle tones and articulations is at once very simple and very complex. Some sounds, like drums, are fairly easy to reproduce because of their short time durations. The longer the duration the more a synthetic recreation will be noticed; there are too many subtle variables that betray it's artificiality. They become less noticeable in ensemble situations.
Besides the subtlety of the instrument itself is the human performance. 'Nuf said here...
It is entirely possible, using sampling, sound libraries and MIDI, to recreate the tone and performance of real musicians. It takes a great deal of knowledge of both the actual instruments you are attempting to recreate and the synth/sampler systems being used. A piano trio (piano, bass, drums) is fairly easy to recreate. Piano is probably the most popular synth/sampler instrument, so a lot of effort has gone into reproducing it faithfully. Drums are fairly easy (cymbals are more complex) and a bass - stand-up or electric - is another fairly simple instrument to recreate. Add a horn section and it's not too bad, but add a solo trumpet or sax and you really have to know your sh...., ah, stuff. I've done jazz guitar solos with a sampler that impressed guitarists, and in live situations some sax work I did on a keyboard had folks looking around for the sax player. But the programming time and practice time ran into the hundreds of hours.
Anything can be recreated faithfully with enough time, money and effort. And that's where the "problems" creep in. If you have a large enough library you can cover any instrument and reproduce it faithfully, but one library can cost over $7,000, and you may need several libraries by different vendors to do an accurate recreation. I know composers with $50,000 invested in sound libraries alone. The time needed for indie composers to build the tracks is usually short, so they rarely have the time to tweak as much as they would desire. In this the "Hollywood" composer has the advantage; with two months to compose and record a score s/he can compose for six weeks and spend two weeks conducting an orchestra of the best film score professionals in the world. An indie composer with a score of equal quality, to recreate the same sonic affect, could take months.
So, yes, a synth/sampler score can be 100% convincing, you've been fooled in films over and over again without ever noticing. It's once again a question of time and budget.
There are lots of composers here on IndieTalk, and thousands more looking to practice their craft. Listen to demos and make a choice.