Dow Theory was first introduced by Charles Dow.Line charts are used with minimum look up period is two months and duration of trend is 2 years.
If the sequence is formed with Higher bottom and Higher Top with it is a Bullish trend.
If the sequence is formed with Lower bottom and Lower Top it is a Bearish trend.
The major criticism was that it could identify only the direction and not the price movement,yet one of the best theories.
Resistance is something which stops the price from rising further.
The support is something that prevents the price from falling further.