Database directions; Roots and ECO?

Example  - ECO: A30  what does it mean? or Root? never use database because I can't find my way around, assume their move orders. Also in a forum a couple days back there was talk of a four move rule, is this if you challage a game it must be played out at least 4 moves before adjourning to move a day?- beginner so need the time.. even though, how they say 'don't thinks so good-don't thinks so much'

4 move rule.  If less than (=) 4 moves made in the game before someone abandons or times out, it doesnt count for ratings.

ECO categorises openings by these combinations of letters and numbers.  Similar openings start the same and are distinguished by the final number.  Its another way rather than using names (some of which are illogical) 

Thanks for 4 move rule, ECO thought would be abbreviation for something and hoped 'A' would be file or some means to know piece placement; it seems not only is there unlimited number of names to openings that I can't remember, thier now listed by letters and numbers I'll never know...SOB!