Eccentric, strange, and peculiar, are synonyms and are all interchangeable. Eccentric sometimes has the connotation that the person may not be very social or that he keeps to him or herself. Peculiar is still sometimes used, but not as much as the other two. Strange is just a generic term that doesn't have any particular connotation other than it's definition. "Weird" is often used instead of strange though. The term freak is a noun and refers to someone who is strange in some way. When you call someone a freak, it has a strong connotation of being derogatory and not very nice. When describing a person's behavior, erratic means that not only is the person's behavior strange, but that it's also unpredictable. It basically means you really can't tell what the person is going to do next, even in situations where a certain behavior would be expected.

Eccentric refers to someone who does not quite behave within the social norm. It can also refer to an object that is set off center from its axis, which is a helpful image. Strange is used in a general way for anything that is unusual from the norm. Peculiar is about the same as strange, except that it can also be used to show an exclusive relationship between two things. Example: the Lemur is peculiar to Madagascar. Freak can be used as an adjective to show that something was very unusual and unexpected. Example: The lightning striking the shed full of fireworks was a freak accident. It can also refer to a person or creature with an unusual physical abnormality. Example: My brother is a freak because he has an extra belly button. Erratic is used to show unpredictability. Example: The drunk driver swerved all over the road. His driving was erratic.

The use of these words probably varies a little with age group, and culture, but to me: 'Erratic' doesn't really fit in with the rest. Erratic just means that someone does things in a somewhat haphazard way. Some people who can't concentrate well can be like that - doing one thing, then another, then back to the first. They wouldn't necessarily be considered in the same category as the others you have used. 'Eccentric' usually means 'a little odd, but harmless'. It is often used for older people, and people like scientists, who consider it unimportant to learn how to react to a situation in a socially acceptable way. They are people who do things in an odd way, or say odd things. 'Strange' and 'Peculiar' are both similar. They convey a sense of odd, and 'possibly a little dangerous'. For instance - you would not allow your children to visit someone who you considered to be one of those unaccompanied. 'you never know what they might do'. 'Freak' is a lot more aggressive and critical. It's often used by younger people, and is sometimes used 'to someone's face' as a nasty insult. The other words above would be generally only used to refer to someone 'behind their back'.