Some words are formed with left and right parts, top and bottom parts, inside and outside parts. These parts are called 偏旁. For example, “好” is comprised of “女” and “子” these 2 left and right parts (偏旁), and the 部首 is “女”.
When loooking up a word using the Chinese dictionary, the first thing one needs to do is to look for the 部首 (if you are not checking via hanyu pinyin). In my 时代高级汉语词典, I can find around 200 部首. 部首 can be the 偏旁 too, but 偏旁 may not necessary be the 部首.
During ancient times, words with similar parts are classified together, and usually they share the same association. For example, “狗”, “猫”, “猪” and “狼” share the same left part - “犭” (反犬旁), and these are words associated with animals. “犭” became the 部首. 部首 are often the first few strokes of a word, but there are exception. Like “学” and “孝”, their 部首 is “子”.
And some of the 部首 can be found at different parts of a word. For example, "心部" can be found on the left in "情" and "怕". It can be found at the bottom in "志" and "息". It can also be found at the bottom in another form in "添" and "慕".
In general, words with the same 部首 share similar association. But some words have lost their original meanings, so they may not seem to have any association with the 部首.
Knowing the 部首 and their associations is one way that can help children remember Chinese words and their meanings. Especially when they are faced with words that look similar, for example - 浪，狼，恨，痕. The 部首 are 三点水, 反犬旁, 竖心旁 and 病字头, and they are associated with water, animals, feeling and illness respectively. This gives them a clue to what the words mean.
I have gathered some common 部首 here for your information: