Yeah, it's weird, Thai and Khmer are very similar, and share some words, but also very different. Khmer isn't tonal, and has really simplified grammar.
They used to share an alphabet, too, but the Thai opted to simplify it (westernise it) while the Khmer opted to keep their original alphabet. Written Khmer is hard for us westerners to deal with because of this. They have lots of vowels and consonants that we don't have (I always struggle with the consonant between 'b' and 'p', because it doesn't seem like there should be any room for another consonant in there).
But English has a consonant between 'b' and 'p' ('pÊ°' in IPA) as well. Just consider the difference between 'ban', 'span' and 'pan'. The 'p' in 'span' is not as forceful as the 'p' in 'pan', and they're actually different consonants.
They used to share an alphabet, too, but the Thai opted to simplify it (westernise it) while the Khmer opted to keep their original alphabet. Written Khmer is hard for us westerners to deal with because of this. They have lots of vowels and consonants that we don't have (I always struggle with the consonant between 'b' and 'p', because it doesn't seem like there should be any room for another consonant in there).