Vietnamese is a tonal language, which means the pitch contour of a syllable is part of the word itself — not emotion or emphasis. The classic teaching example is ma, má, mà, mả, mã, mạ: six syllables that share letters and differ entirely in meaning.
Northern Vietnamese is usually described as having six tones; many southern accents merge two of them, leaving five in practice. Neither is more "correct." They are accents of the same language, the way English accents differ without one being the real English.
Vietnamese is written today in chữ Quốc ngữ, a Latin-based alphabet with diacritical marks. It was developed in the 17th century with the involvement of Portuguese, Italian, and French missionaries — Alexandre de Rhodes is the name most often cited — and became widespread in the 20th century. Before that, Vietnamese was written with chữ Hán (classical Chinese characters) and chữ Nôm, a locally invented character script used to write the Vietnamese language itself.
That history is why Vietnamese looks familiar to a reader of European languages and yet carries an enormous layer of Sino-Vietnamese vocabulary underneath, the way English carries Latin.
One more thing that surprises new learners: Vietnamese has no true universal "I" and "you." You address people using family words. That is the subject of lesson 6, and it is one of the most culturally revealing things about the language.