"The man is similar to the woman."
Translation:Der Mann ist der Frau ähnlich.
Many adjectives (in German and in English) can be linked to nouns. In both languages, many of those adjectives use prepositions to indicate the relationship they create. For example,
Die Truppen waren zum Einsatz *bereit. - The troops were *ready for action.
The adjective bereit/ready governs Einsatz/action through the preposition zu/for.
In German, however, adjectives sometimes use case instead of prepositions to indicate their link (this is not possible in English, which must use either prepositions or fixed word order). Some govern the accusative (i.e. Ich bin den Lärm nicht gewohnt - "I'm not used to the noise"), some the genitive (i.e. Sie war seines Erfolges froh - "She was pleased with her success"), and some the dative, like ähnlich.
As for the word order, some adjectives can precede their object, some cannot. Ähnlich happens to be one that can, so "Der Mann ist ähnlich der Frau." is okay.
"Als" has other uses (never a preposition, though). See, for example, http://www.thegermanprofessor.com/wort-der-woche-german-als.
Disclaimer: I'm not a native speaker. My source is Hammer's German Grammar and Usage by Martin Durrell.