In English, the letters A-Z are commonly referred to as the alphabet. They are the building blocks of words and underpin children's development of skills in reading, writing, and spelling.
The letters have a name. The letter "b" for instance, we pronounce as "bee." The letters also make sounds within words. The letter "b" (bee) makes the sound "b" as in bat, except at the end of words like thumb, when it makes no sound at all. Some letters, like "g," make different sounds as in goat and age, and then make a whole new sound when in combination with another letter as in cough. This is what makes our language so confusing! We also write the letters two ways, as uppercase (capitals) or lower case. When I'm teaching kids I make sure they know about letters, sounds, uppercase, and lower case when the need arises. I prefer to introduce the letters formally in lowercase format first. My reasoning is that children are more likely to see more lower case letters in books and the environment.
The best way to help kids learn the alphabet is, in my opinion, via fun and games. Here are some resources you might like for your youngsters.
I Spy: One of the oldest games, this is nevertheless a great one as it needs no equipment and can be played anywhere. If you see a piano for example, you would tell your child, "I spy with my little eye, something beginning with "p." Your child tries to work out what it is.
Alphabet Books: Look for printed alphabet books with pictures that will interest your child, and large simple letters. Children will often learn such books by heart, an important stage in the road to real reading. Also consider making your own alphabet books, perhaps from photos of your child's toys or of family members.
Making Letters: Tracing, painting, and modeling letters from play dough are all excellent activities to help children focus on the way a letter is made. Keep it fresh by painting outside with a wall painting brush and watercolor or trying other different paints, brushes, and surfaces. I like to teach kids from the start how to make a letter according to the correct model for their school, but some people think this is me being a fuddy-duddy.
Hide the Letter: It's great to have letters printed on cards, or plastic letters or even foam ones for the bathtub. Incidental remarks like "Oh, what is that 'W' doing under the tap?" or "Mummy has hidden the 'B.' Can you find it?" take only a few seconds, but constant repetition means that over time it aids letter recognition.
Magnet Letters: Having magnet letters attached to the fridge or a simple metal baking sheet means you have a ready source for simple games of hide-the-letter, word making, word muddling, etc. Alphabet blocks are wonderful too. Make it a habit to leave little messages or muddled up words on the fridge each day, and your child can do the same.
Toy Letter: One simple way to encourage letter recognition is to tape letters to toys or blocks. LEGO Duplo is great for this, especially once kids have moved on to making words from letters, because the blocks link together. Try incidental questions like, "Where is the D truck?" or even games where you are the traffic controller and your youngster needs to bring certain vehicles of the alphabet to designated places. If your child has a large doll collection, try matching letters to the dolls' names. Build towers in turn with toddlers, naming the letter facing you as you do.
Letter Sentences: Try making sentences where each word mostly starts with the same letter, e.g., Andrew Ant accidentally ate an apricot.
Allison McDonald of No Time for Flash Cards has 25 excellent alphabet activities you might like. This Reading Mama has lots of free printables for preschoolers, including many that are alphabet-related.
ABC Spy HD is a free iPad app I've reviewed that encourages children to use the iPad built-in camera to find and photograph objects. The letters of the alphabet are clear and easy to see. I believe that kids are more likely to remember something significant from their own lives that's linked to a letter of the alphabet than one an adult chooses.
Another app I've reviewed, ABC Wildlife, can also be used as an alphabet book.
One I haven't reviewed yet but like the sound of is Jamaroos Musical ABCs.
Common Sense Media lists some more alphabet apps.
Alphabet Resources Online
Starfall is an excellent free website with useful alphabet activities.
ABCya.com is a game site with fairly low-key ads. It offers several alphabet games:
Alphabet Slider Puzzle
These games are suitable for young children (perhaps kindergarten and first grade) who are ready to learn the alphabet and can use a mouse. They are simple to play and offer practice with letter matching, recognition, sound and letter correspondence, alphabetical order, etc. I like that there are options for both lower and uppercase letters.
Nick Jr. has ads you can skip. It has a game called Letters with Oobi that really stresses the letter/sound connection.
Learn English Kids has Alphabet Antics, a game with both upper and lower case letters. Children must help the monkey by matching letters to coconuts.
Finally, take a look at Read Write Think's Alphabet Organizer. It's a tool that enables children to create their own little alphabet books with either multiple words for each letter of the alphabet or one word and an image.