If I told you that your Korean language vocabulary can be improved by learning a writing system that most Korean people don't write with, I wouldn't be surprised if you raised an eyebrow or two (or thought that I was completely crazy!). But let me give you some context... and it will all make sense.
Modern Korean language is written in Hangeul (한글, lit. Korean alphabet), a writing system designed specifically for Korean language in 15th century. It is one of the only scripts in the world that were made by humans and not created by communal use and organic changes. If you opened this link, it is likely that you already know hangul. If you don't, I will quote the Great King Sejong who is attributed with its creation: "A wise man can acquaint himself with them before the morning is over; a stupid man can learn them in the space of ten days". Putting aside slighly awkward phrasing (I definitely needed over 10 days, thank you King for calling me stupid!), the point is clear: Korean writing system hangul is designed to help anyone achieve literacy, due to its simplicity and limited number of characters.
But if hangul was invented in 15th century, how did people write before?
Enter Hanja (한자 / 漢字 ), Chinese characters used in Korean! In Chinese they are called Hanzi, and in Japanese Kanji - even though they all stem from the same source, individual countries adapting them changed both the pronunciation and sometimes the shape of the characters. Korean Hanja used to be main script used in Korea, alongside with Chinese language for scholarly and administrative purposes. Unlike hangul, there are thousands and thousands of Chinese characters, making it difficult for large population to achieve literacy. Even though Korean language switched to hangul, due to centuries of language contact with China, there is a large proportion of Korean vocabulary that is of Chinese origin. Those words are sometimes called Sino-Korean (meaning "Chinese-Korean") and are fully integrated and, at a glance, indistinguishable from native Korean vocabulary. It is approxomated that around 50% of all Korean words are of Sino-Korean origin. That means one thing for you: they may be written in hangul today, but they have hanja in the background!
It's easiest to explain how hanja works through an example. Let's take days in the week - have you noticed that they all end in - 요일? So, 요일 must mean "week day", right? Well, you would be right, but there is more to it. 요 means "week (day)" and 일 means "day" or "sun".
Now we can understand why 일요일 is a Sunday! It's no coincidence that 일 is repeated twice. It means "Sun" in the first position, and "day" in the second position. If you're wondering how do we know this, it's precisely because of hanja in the background. 일요일 can be written as 日曜日 in hanja, where you can clearly see the repeating character. Not all characters with same sound in hangul have the same hanja behind them. For example, sound 일 has at least 16 different hanja characters it can be written with, which you can check in a Hanja dictionary if you're curious. Don't worry, you don't have to learn all of these, and certainly not as a beginner - but it would be good to know that two most common meanings behind the sound 일 are "one" and "day/Sun".
But how can you tell which hanja makes sense to learn, as there are thousands of characters and not all are productive (=used in many words)? I can recommend an exellent free online source, Hanja List. The characters are arranged in order from most common ones to least frequent ones, and each individual character provides vocabulary ranked in frequency as well. Very useful place to start!
Note that hanja usually replaces one character in hangul. For example, hanja for woman (女, read 녀 or 여) is used in words like 소녀 (少女, lit. little girl), but it does not replace the entire word for woman (여자 has no hanja).
I said at the beginning that Korean people don't write hanja nowadays. Well, that is not entirely true. If you've visited Korea, maybe you've seen aformentioned hanja for woman (女) or its counterpart man (男) on toilet doors. By the way, did you know that toilet (화장실) is written as 化粧室, meaning "the make-up room"?
If you ever had most famous Korean ramyeon, Shin Ramyeon, you must have noticed a hanja character 辛 on its packaging. 辛 or 신 means "매울" or spiciness, which makes sense if you ever tried it. You can see it in many places on the streets of Korea, whether it is to appear more traditional, or to represent their brand in a creative way. For example, the LED store shown below is using a hanja for "light" (빛 in native Korean and 광 in Sino-Korean reading). What's the catch? Together it's pronounced as 광화문 or Gwanghwamun gate in central Seoul, so it's refering both to the location and the type of business. Hanja can make your language expression more creative!
Products and store names aren't the only usage for hanja in modern Korea. It is sometimes used in newpaper and specialised/academic books, but perhaps more importantly - it's a base for many names and last names. Almost all Korean last names have hanja behind them. You probably know that the most common Korean last name is Kim, followed by Park and Lee. Kim or 김 is "gold" (金), Park or 박 is 朴, which can mean a type of tree (nettle tree; but its etymology is unclear and very complex), and Lee or 이 is 李, a plum - and the same character is used by over 90 million people in China as wel, hence the different spellings across the Asian countries: Li, Lee, I, Yi, Ri, Rhee. Given names sometimes have hanja and therefore a meaning too. For example, 은지 can mean "kindness" or 정호 can mean "proper and excellent". There are many combinations, and names with a same sound can have different meaning if they use different hanja. Try asking your Korean friends if their names have a meaning!
How you can use hanja for your benefit? I would start with identifying simple characters, and recognising them as you learn words. Hanja List website, which I mentioned earlier, is a good free resource, but it has very brief information: just a word list and no additional explanations. You might want to look into some of the textbooks such as Your First Hanja Guide by Talk to me in Korean, or an excellent partially free website How to study Korean with their Hanja lessons. You don't need to be able to write hanja by hand to get vocabulary benefits from it. It's a nice skill to have, but unless you are also studying Chinese and Japanese, it is probably not necessary. Just the awareness of which character is in the background of a word can help you make sense of Sino-Korean vocabulary.
What are your experiences with hanja? How do you study? Do you know some helpful resources? Feel free to share your story in the comments.