The electric bass was born very soon after the invention of electric guitars in the middle of the last century. Electric basses look pretty much the same as electric guitars. The main differences? They have a longer neck and usually only four strings. The thick strings of a four-string bass have the same names as the four lowest strings of a guitar (E, A, D, G). Because they are thicker and longer, they sound twice as low. An octave lower, in musical terms.
While the 4-string bass is the most common model, it is not uncommon to find 5-string basses: usually an extra low B string, more rarely a high C. Another possible variant is the fretless bass, with a smooth fingerboard like the one on the double bass.
In all the above cases, the instrument is almost always of the "solid body" type, like the most common electric guitars. However, there are some rare "hollow body" models, such as the one used by Paul McCartney with the Beatles.
Like its electro-acoustic guitar counterpart, the electro-acoustic bass is distinguished from other bass guitar models by the amplification system: this is often a piezo system built into the bridge, unlike all other bass guitar models which use either one (P-model) or two (J-models and Fusion) pickups comparable to the magnetic pickups on electric guitars.
Some basses, notably the Fusion models, have what is known as a "neck-thru": unlike the traditional set-neck or bolt-on models, this neck runs right through the body. The advantage is twofold: on the one hand, the two ends of the strings are fixed on the same piece of wood, which favours sustain. On the other hand, access to the high notes is greatly facilitated, it is thus possible to obtain a range of two full octaves (24 frets) where traditional models are limited to 20 frets.
Another instrument is the electric double bass. A double bass resembles a large violin, played upright. The biggest disadvantage of the instrument is the enormous body. On an electric double bass this has been removed. The electric double bass is always fretless, just like the normal double bass. It is played exactly like a double bass, with the fingers ("pizzicato") or with a bow.
Like electric guitars, bass guitars of all types, even electro-acoustic basses, need an amp, the quality of which will determine the final sound. A word of advice: do not use a guitar amp with a bass. You risk irreparable damage to the speaker, which is not designed for such low frequencies.