Sexual lubrication or arousal fluid is blood plasma that is squeezed from the cells in the vaginal walls when you get aroused and experience vasocongestion (swelling up). It will often differ in different phases of your cycle and your life and may also depend on how hydrated you are.
Sexual lubricants are often hidden as 'personal lubricants' and can normally be found with the condoms, pregnancy tests and yeast treatments in the same isle of your store. Some also have a warming effect which is said to make the clitoris and penis much more sensitive and arousable.
If you are trying to have a baby it is important to have a sperm friendly lubricant. Normal sex lubricants and saliva traps sperm (it does not kill the sperm but makes it difficult for it to escape up the cervix). Pre-seed is one such a sperm friendly lubricant.
As W0LF said, water based is by far the most preferable ones. (The opposite is glycerin/petroleum/oil based lubes that are just plain nasty to condoms and difficult to wash off) If a water based lube dries out, you can add just a few drops of water and you are in business again. Water based lubes also plays well with sex toys and condoms if you are not trying to conceive.
There is also a spot at the top end of your vagina (called the 'deep spot' or 'X spot') that researchers are thinking can control lubrication. If your partner have long enough fingers you can try it. It is between the cervix and the top front (belly side) of your vagina. Researchers have seen a massive increase in lubrication when this area is stimulated.
If he is using a condom, a squeeze of lubricant inside the condom can make it much nicer for him. It will make it feel much more natural for him and you and it will conduct heat much better.
Some lubes are also flavored but I am a bit suspicious of it. Any sugars in your vagina will cause yeast problems so I just don't trust these.
Best of luck with your quest! You will be amazed by how much fun a good lube can add to your sex life.
