Students taking their first steps usually approach teachers with the following question: 'is it better to learn on an acoustic or an electric guitar?' The short answer to that question is 'they are different instruments, and your choice should depend on only one thing: which guitar you would like to see yourself playing.'
There are a number of myths or common misconceptions among starting players, namely that acoustic guitars will make your fingers stronger in the very early stages of practice, nylon stringed guitars can only be used for classical music and electric guitars are easier to play. Though these statements are somewhat based on real facts, they can be misleading and you should not base your choice on them.
Your first thought when you pick up a guitar should be about the music that you want to do with it. If you choose an instrument thinking it’ll be easier than others you will be in for a surprise: even the first steps in guitar playing demand practice time: your fingers will probably get sore when playing either acoustic or electric and you’ll soon come to face technical challenges when practicing a simple tune.
So the main factor in choosing what type of guitar you’ll be playing is the music you want to make with it. It all comes down to what attracts you and motivates you to play, and you should really pick an instrument that you will look forward to practice with. Consider the music you listen to and you’d like to play and make a choice that won’t leave you stuck with the wrong type of guitar.
All of the above considered, there are indeed physical differences between types of guitars.
Electric guitars can be solid body, semi hollow, or hollow. They use light gauge strings (easy to press down) and, as long as they properly maintained, you’ll find that your hand runs smoothly through the neck. Once plugged into an amplifier, a light touch on the strings will be enough to create sound, making playing less tiresome. This can be convenient for beginners that tend to lose interest if their first experience is not successful. Though smaller in body size and usually thinner in neck, electric guitars can actually feel heavier than their acoustic counterparts. One important thing to keep in mind is that an electric guitar isn't a complete instrument without an amplifier (and that makes for some budget considerations). Some people find that the preparation it takes to play electric guitar (the effort of taking out a cord, plugging the guitar into an amp and turning it on) eventually leads to not practicing that much as they’d like to.
Acoustic guitars have a bigger body and slightly thicker necks. There’s no mics here, as the guitar itself projects the sound: some people feel this gives the acoustic guitar a more ‘intimate’ feel, as the final result depends only on the guitar and the performer. Because of the type of strings they employ, acoustic guitars demand a little more strength and control in fingering and beginners tend to be discouraged by this fact (which presents itself in all its splendor when trying to do barred chords). It is important to keep in mind that this is merely a stage in learning.
Sometimes acoustic guitars will have electronics fitted (these instruments are usually called electro-acoustic), allowing to plug into an amplifier and reach high volumes without resorting to external microphones. This makes them a little bit more expensive, but it's an investment that makes the guitar usable in band / live show situations. As these guitars need not to be plugged in to work properly, they can be easily picked up at any moment, making them ideal for spontaneous playing. They come in a lot of varieties, with two main types: steel-stringed acoustic guitar and nylon-stringed (also known as classical guitar).
Classical guitars employ nylon strings, which are easier to press down than steel ones. They have a very characteristic sound, easily distinguishable from the acoustic guitar, and more fitting to specific genres. Their necks are wider, and the action is usually higher.
The action is what guitarists call the distance from the strings to the neck. In general terms, it is recommendable to have a low action (that is to say, the strings near the wood), as this reduces the strength needed by the fretting hand. The more relaxed the left hand can be, the easier it becomes to have a clean tone, develop speed, and perform big stretches. Electric guitars will commonly have the lowest action, while Spanish or classical guitars will tend be have a higher one. An excessively low action can make the strings buzz against the guitar itself, which is something that should be corrected by a luthier. A high action may have some benefits in terms of sound, and it allows the use of a slider, but it comes at the price of not being able to perform at the full potential of the left hand.
It is easy to find super-cheap acoustic guitars, and some teachers argue that budget instruments are a good choice if the student is not quite sure about sticking with playing the instrument. The reality is that budget acoustic instruments are usually so hard to play it becomes a factor of discouragement for the student, so if money is really an issue it would be wiser to invest in a cheap electric+amp combo. The instrument may be far from the best, but at least it will be easier to play.
Though guitar players often specialize in one type of guitar, most of them will actually be familiar with both electric and acoustic instruments (is not uncommon for professional players to own a variety of them). This is just natural: different types of guitars offer different possibilities, and as time goes a desire to experiment with other sounds and develop new skills will grow. Making your initial choice is important because it determines your first experience with the instrument (and maybe with music itself), but by no means is decisive: a good number of well known musicians started with an instrument and ended up mastering a totally different one years later.
It doesn't matter what type of guitar you choose, always make sure that the guitar you’re buying is correctly adjusted and prepared for playing. If you can, bring someone else with you to the store and examine the instrument before buying it: try it, go though the high and low registers and make sure that your hand is always comfortable. A poorly crafted instrument can seriously hamper your progress and make you feel like you are not getting better.