Some of the most confusing words for non-native speakers, and sometimes for native speakers as well, are the words "yeah" and those similar to it. There are at least four other words with similar spelling that are used commonly in English: "yah," "ya," "yay," and "yea." The previous term and "yah" are quite similar in usage, pronunciation, and meaning, expressing an affirmative. People typically use "ya" as a slang version of "you," "yay" as an expression of happiness, and "yea" as an affirmative vote or for more archaic purposes.
Yeah and Yah
"Yeah" is the most common of these words, and seems to have originated sometime around the early 20th century, likely in the US. It is an adverb, and speakers often use it simply to mean "yes." People can use the term as a one-word answer to yes-or-no questions, such as "Do you want to go to the park on Saturday?", to which someone could simply reply "Yeah." This word likely derived from either the word "yes" or the word "yea."
Many people use "yah" as a more modern, slang spelling of the word "yeah". People often use it in instant messaging or texting, and it is likely just a shortening of the slightly longer word and indicates agreement in much the same way. The pronunciation can be the same, or it may be pronounced more like "yaw." An older use of the word "yah" was as an interjection that usually showed some sort of negative feeling about something said, often insulting it, or sometimes demonstrating impatience.
Ya
The word "ya" can be used as an even further shortening of "yeah," typically in texting or online communications. More often, however, it is used as a synonym for "you," with a dialect pronunciation. A speaker might use it in a sentence such as, "How are ya today?" In this sense, the word is pronounced as "yuh," rhyming with "duh." One archaic usage of "ya" was as a synonym for the word "yea," but this is seen only in very old English writing.
Yay
The word "yay" is usually used as an interjection and exclamation to show joy. A person might say something like, "Yay! My new kitten finally arrived!" The word is likely derived from "yea," meaning simply "yes," but shifted spelling over time, perhaps to indicate excitement. "Yay" has the same pronunciation as "yea" and rhymes with "hay." It can also be used as a placeholder to denote physically representing something, usually height, in sentences such as, "The plants were about yay high."
Yea
The word "yea" is the oldest of the group, and it is likely that "yeah," "ya," and "yay" all derive from it. "Yea" goes back more than a thousand years, from the Old English "gea" by way of Middle English "ye." It has a number of meanings, but all are roughly affirmations, much as "yeah" is.
"Yea" can be used as a synonym for "indeed," as in the sentence, "Yea, it did come to pass." It can mean something like "not just this, but also this," as in the sentence "a large, yea, a titanic wave came crashing in." Someone can also use it simply to mean "yes," as in the sentence "Yea, I will do it." In modern usage, however, speakers typically use it only in voting, where it indicates an affirmative vote, in contrast to "nay." The word "yea" has the same pronunciation as "yay," rhyming with the word "may."