Lubricant can make most kinds of sexual activity more pleasurable, and for certain kinds of play, some kind of lubrication is a necessity. What kind of lubricant you choose will depend on what kind of sex you are planning to have, what formula or ingredients work best with your body and your toys, and whether or not you need the lube to be latex-compatible.
All lubricants that are sold for sexual use pretty much fall into four categories: water-based, silicone-based, hybrids, and oil-based.
Water-Based lubes are the most common and the most versatile kind of lube. They are compatible with latex condoms/dams/gloves and with all kinds of sex toy materials, as well as being generally friendly for vaginal, anal, and oral sex. Water-based lubes tend to feel the most natural (our body fluids are mostly made of water), though they will evaporate and absorb into the skin as you use them, meaning that you may need to reapply or re-wet them periodically to keep them slippery.
A recent addition to the water-based lube market are Aloe vera-based lubes. They are quite slippery and have a gel-like quality. Aloe-based lubes are water-based lubes-- after all, the leaves of the aloe-vera plant are mostly made of water! Aloe-based lubes are compatible for all kinds of sex, and they are condom/dam/glove and toy friendly. Aloe has the added benefit of being a natural moisturizer, so if you or your partner is feeling dry on a regular basis, aloe-based lubes might help soothe that discomfort.
Silicone-based lubes feel incredibly slippery, and they do not readily absorb into the skin the way water does. As such, they often last longer than water-based lubes. Silicone does not wash away with water alone, making it good for underwater play-- though this quality also makes silicone harder to clean up afterwards. You'll need soap and water to wash it away, and sometimes it can stain fabrics. Silicone lubes are compatible with latex condoms/dams/gloves, but will damage silicone toys-- stick with water-based lube if you are using silicone toys. Silicone can also be difficult to wash off of porous toy materials like TPR or elastomer, leaving behind a residue. When in doubt about your toy material, it is best to stick to water-based lubes!
Hybrid lubes usually refer to formulas that combine water-based and silicone-based ingredients. This is usually done to create the natural feel of water-based lubes while gaining the added longevity and slipperiness of silicone. Silicone/water hybrid lubes are compatible with latex condoms/dams/gloves, but whether they will damage silicone toys will depend on how much silicone is in the formulation. When in doubt, read the label or ask a reputable, knowledgeable source about toy material compatibility. You can also spot test the hybrid lube on your toy in a small area, such as on the base of a dildo, to see if the lubricant causes any damage.
Oil-based lubes are a bit less versatile than other lubes, but they have their uses. They are not compatible with latex condoms/dams/gloves (though oils are safe with polyurethane and nitrile barriers). It is difficult for the vagina to clear out oil-based lubes, and residual oil in the vaginal canal can harbor bacteria and cause health issues (different bodies will react differently, and it might even depend on the type of oil for any given individual). All that said, oil-based lubes can feel rich and long-lasting. They can be used for external masturbation/stroking, or for anal play if a latex condom is not being used. Oils are often not compatible with certain soft toy materials like TPR, latex, or elastomer, so check with manufacturer recommendations before using oil-based lubes with your toys. 100% silicone toys are typically compatible with oil-based lubes.
OTHER LUBE TIDBITS
There are lots of different ingredients in lubes, but some of the most common ingredients can cause problems in some users. If you or your partner are having trouble with irritation, here are some lube ingredients you may want to avoid:
Glycerin or Glycol: Used to make lubes feel slippery, but some users report links to yeast infections, irritation, and other imbalances in vaginal flora. That said, other users find they have few issues with glycerin. Our bodies often have very individual tolerances for certain substances, and those tolerances can vary over time.
Parabens: Used as preservatives, these can cause skin irritation in some users. Parabens have come under scientific scrutiny as possible endocrine disruptors, and some studies have linked them with cancer. The research in these areas is inconclusive, but some people feel more comfortable choosing lubricant options that are paraben free.
Warming/stimulating/intensifying lubes: While these lubes can be enjoyable for some people, the ingredients in these types of products can cause itching, burning, and yeast infections. Use caution when trying stimulating lubes if you have sensitive skin.
Numbing/prolonging lubes: Marketed to "relax" or numb certain areas such as the throat or the anal canal, or to prevent premature ejaculation. In general, such products work by removing the body's ability to feel anything-- good or bad. Because they deaden pain (our body's warning signal that something is wrong), such products can be potentially dangerous.
Ingredient Purity: When choosing a lubricant, look for reputable lube manufacturer who makes their product in the USA or Europe, as the possibility of chemical or toxic impurities is far less common.