Biochemistry involves the chemical makeup and molecules of an organism. Though carbon is the most suitable of elements to be the basis of an organism's biochemistry, other elements can be substituted.
The majority of life in the universe is carbon-based, because carbon has four valence electrons - it is capable of bonding to four other atoms, and its stability to hold these atoms together allows it to form a wide variety of complex molecules. The most commonly bonded elements are oxygen, nitrogen and hydrogen. Other elements found in carbon-based life forms include phosphorous and sulfur.
Carbon based life will form four basic macromolecules which are used to carry out life processes. Proteins which form the structures of living organisms, nucleic acids which contain genetic information, carbohydrates which store energy used by living cells, and lipids which store energy for more extended periods than carbohydrates.
Methane-based life is a hypothetical type that can survive on a planet like Saturn's moon, Titan, a world filled with hydrocarbon lakes. Technically, such type of life is an alternate form of carbon-based life, but they consume a different set of molecules. For sustenance, methane-based life would intake hydrogen, acetylene and ethane, while exhaling methane. Such creatures will have to survive temperatures as low as -290 F, because the formation of such a climate is only possible at distances which are outside the habitable zone for garden planets.
Appearance-wise, a methane life form would resemble a carbon-based creature, but multicellular life forms will be extremely limited when it comes to size. As a result, they must have a small surface area or else they would freeze up in their planet's chilling environment.
Titan-like climates could arise easily around a rocky planet around a Class M star or a moon orbiting a gas giant in a Class G system.
Ammonia-based life forms use ammonia instead of water as a molecular solvent. Water and ammonia have plenty of analogues with one another. Both substances can dissolve salts and small polar molecules. However, ammonia is not as efficient at forming hydrogen bonds as water is resulting in life that is far less adaptable.
In order to remain stable, an ammonia based life form would instead prefer much colder environments between -78 C and -33 C, the temperature ranged where ammonia is in a liquid state. However, higher pressure environments will allow ammonia to remain in liquid state at much higher temperatures.
While gas giants and even some terrestrial planets could potentially harbor ammonia-based life, finding a sapient ammonia being is incredibly rare because of all the precise conditions required for them to thrive. An ammonia-based life form could not survive in a standard nitrogen-oxygen atmosphere as not only is water dangerous to them, but their bodies would split open due to the low pressure.
Silicon, like carbon, also has four valence electrons and can form a variety of molecules as well. Instead of long chains, it will form a crystal lattice, resulting silicon-based life appearing as crystals. Silicon-based life lacks the agility of carbon-based life as a result. Silicon-based life will feed on electromagnetic radiation and communicate with one another via harmonic resonance. As a result, they are not likely to have the same sensory organs found in carbon based life.
Germanium also contains four valence electrons. Just as silicon based life appears crystalline, germanium-based life appears to be made of rock. Germanium herbivores will likely feed on rocks and minerals.