For 100, cien, a shortened version of ciento is used. The numbers 200, 300, etc., are written as all one word and the plural cientos is used (doscientos). Note that unlike English, the word hundred in Spanish becomes plural after one hundred, giving the literal meaning of two hundreds, three hundreds, etc. Also note the slight variations of spelling.
For all other compound numbers y (and) is used only between the tens and the units (for numbers ending in the 30s and above). Ciento is used for compound numbers in the 100s. When compounds of cien are used to describe feminine nouns, the feminine form is used. Finally, the word mil (thousand) is never plural, but millón (million) and billón (billion) are.