rhythm                                                                                                                                                                                      dots                                                                                                                                                                                                                                         
BAR, BAR LINES, REPEAT SIGNS

BAR or MEASURE
A bar (or measure) is a segment of time defined by a given number of beats, each of which are assigned a particular note lenght.
 A piece of music consists of several bars of the same length.

Number of beats in each bar is specified at the beginning of the score by the top number of a
time signature (such as 3/4), while the bottom number indicates the note value of the beat (the beat has a quarter note value in the 3/4 example).

The word bar is more common in British English, while the word measure is more common in American English, although musicians generally understand both usages.
2bar                3bar                 4bar
2 beats in a bar        3 beats in a bar          4 beats in a bar
In each example we can see different bars with strong and weak beats

BAR LINES
  A bar line is a vertical line that separates each bar or measure and divides the stave equally.
There are different types of bar lines:
singlebar BAR LINE: it divides the stave into bars.
                doublebar DOUBLE LINE: it is usually used when there is an important change in the music.
                                      endline  DOUBLE BAR LINE: it indicates that the end of the piece of music has been reached.

REPEAT SIGNS

 Repeat sign is a sign that indicates a section should be repeated.

repeat1 

If the piece has one repeat sign alone, then that means to repeat from the beginning, and then continue on
(or stop, if the sign appears at the end of the piece). 

repeat

A corresponding sign facing the other way indicates where the repeat is to begin.
When you reach the second sign, go back to the first and repeat the music.
These are often accompanied by first, second and even third endings.

endings

When a repeat calls for a different ending, number brackets above the bars indicate which to played the first time (1), which to play the second time (2), etc. These are called "first-time bars" and "second-time bars," or "first and second endings".

D.C.
Da Capo is a musical term in Italian, meaning from the beginning (literally "from the head"). It is often abbreviated D.C.

D.S.
This is a directional marking. It means 'Del Signo'. When you see this in music, you must go to the sign (below). This marking may also be accompanied by 'al coda' or 'al fine'. These mean 'Go to the sign, from there go to the coda' and 'Go to the sign, from there go to the end' respectively.

signo This is the Sign. From here you play to the coda or the end or wherever the Del Signo directs you.

              coda This is the coda sign. It marks when to go to the special ending, or coda
                            Usually you won't go to the coda until after a D.S. al coda.

CODA

(Italian for "tail", plural code) is a term used in music in a number of different senses, primarily to designate a passage that brings a piece to a conclusion.

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