-ssi (씨) is like Mr. or Mrs. It is the formal way of addressing someone when you are talking to or about them, such as 이마씨 반가워요! - (Ima ssi bangawoyo) - "Nice to meet you, Ms. Ima!" or 이씨 만났어요? (Ee ssi mannasseoyo?) - "Have you met Mr. Lee?" On the other hand, -a or -ya is a more informal way of getting someone's attention when you are talking to them. For example, 형 아, 지금 뭐 하는 거야? - (Hyeong ah, jigeum mwo haneun geoya) - "Hey bro, what are you doing?" in a kind of "hey, stop that!" tone or 선우 야, 어디에 가? - (Sun-woo ya, eodi e ga?) - "Hey Sunwoo, where are you going?"

never called sum1 with those. they seem rude

There's a difference in level of politeness/formalities. Listen to this lesson. They talk about it somewhere in it. http://www.talktomeinkorean.com/lessons/level-3-lesson-27/