There are several systems of Romaji, with the Hepburn system being the most widely used. These characters represent sounds, specifically syllables.
These words often have a string of kana called okurigana that come attached to the word. Kanji What is Kanji? So it is not rare to see some sentences that use all three systems or more! Colons and semicolons are available but are not common in ordinary text. When written on the page, each character is given exactly the same amount of space, no matter how complex it is.
Kanji is used for words of both Japanese and Chinese origin, as well as many Japanese names. Why Kanji? Compare this to the Korean alphabet which has 14 consonants and 10 vowels. In addition, this will help you learn new vocabulary, which will often have combinations of Kanji you already know.
LingQ makes reading hiragana and katakana easier as words are often spaced out which will help you distinguish between where a word starts and ends. In fact, Korea adopted their own alphabet for Korean to greatly simplify their written language with great success.