Lime is green and more pungeant than a lemon (which would be yellow!). Furthermore lime is more sour and is so much more smaller than lemon. There are many different varieties of lemons, but it's hard to tell the difference between them. Lemons are very useful citrus fruits. 

Although they are often too tart for eating, their juice is used to flavor everything from fish to salads and fruit desserts. Lemon juice, low in calories, adds flavor and "zip" to foods and is a good substitute for salt, salad dressings and sauces. The juice from 6-8 lemons makes about 1 cup of fresh lemon juice. When it's hot outside, make fresh lemonade. It's a great way to rehydrate yourself and quench your thirst! 

The best lemons are heavy for their size and have a fine-textured skin. Deep yellow lemons are usually more mature than light yellow ones and not quite as acid. One lemoncontains 35% of the vitamin C your body needs every day. 

Limes have a flavor similar to lemons, but are more fragrant and less acid. Limes are also important for their juice. When selecting limes, choose those that are firm and heavy, this means they have lots of juice. Depending on the type and size of the lime, it will take between 6 and 9 to make 1 cup of fresh lime juice.

Switching between lemons and lines in recipes is mainly an issue of personal taste. Where a dish is a "pure" lemon or lime flavour (such as a lemon cake) you can switch from one to the other for variety, but where a lemon or lime is an intrinsic part of a dish it is less advisable to change as it may affect the end result. 

The Persian lime is the variety most commonly found in supermarkets around the world. This lime is slightly lower in vitamin C and slightly higher in natural sugars than lemons, making it a little sweeter than lemons. The difference is quite small but we would suggest adding the juice of either lemons or limes a little at a time and letting your taste buds act as a guide.

Lemons do however tend to yield more juice than limes (as a rough guide a lemon produces about 4 tablespoons of juice and a lime only about 2 tablespoons) so if the recipe asks for the juice of a whole lemon or lime than you will need to adjust accordingly.

The zest of lemons and limes contain essential oils which are very strongly flavoured, so if a recipe contains a lot of zest then you will see a more pronounced difference in the flavour of a dish. When using the rind of lemons and limes make sure that you only use the coloured zest on the outer surface as the white pith part is quite bitter.

Most citrus fruits are coated in a thin layer of wax to reduce damage to the skin while the fruits are in transit. Although this wax is edible it is a good idea to wash the fruits in hot, soapy water and give the skins a bit of a scrub before zesting as this will help to remove some of the wax. Rinse the fruits then dry the skins thoroughly before zesting. Microplane-type graters are particularly good for grating zest from citrus fruits.

Source: http://www.nigella.com