An electron is a negatively (-) charged subatomic particle. In a neutral atom they exactly balance the positive charge(s) of a the proton(s).
 In chemistry
Electrons are arranged into "shells" of increasing energy around the nucleus, and within each shell they are arranged into orbitals. The highest energy electrons in the outermost shell of an atom are known as the "valence" electrons, and these are the basis of all chemistry. When atoms exchange or share valence electrons, and thus rearrange themselves, a molecule is formed. The formation of molecules, and changes in them, are called chemical reactions. To particle physicists, the electron is too large to bother with, so they like to smash them against positrons to see what happens.
The term "particle" is unfortunately a misleading misnomer, for in fact the electron does not on its tiny level resemble a spheroid. Indeed, it doesn't "look" like anything, for visible light consists of waves of electromagnetic radiation whose wavelengths are considerably larger than the packet of negatively charged mass whose behavior in exchange with counterparts surrounding other atoms constitute what are termed chemical bonds.
An electron is an example of "particle-wave duality". Originally, the electron was thought to orbit the atom's nucleus like a planet would orbit a star. This model fails, however, since the electron's orbit would decay, and it would eventually crash into the nucleus rather than stay stable. After it was shown that an electron fired through a fine grating would diffract like a wave, it was re-described as a wave. At the same time a good boffin showed conclusively that the electron was a particle, as exhibited by the photoelectric effect. As a result, an electron is sometimes (quite informally) referred to as a "wavicle".
It's important to note that this doesn't mean that it is a particle and a wave, it's something else entirely, that we cannot fully comprehend in our macroscopic environment, that has the properties of both particles and waves. This is the basis of most quantum mechanics.