In Eto, time and direction, etc. are represented by a combination of 10 signs called 'Jikkan[十干（じっかん）](literally, 10 Stems)' and 12 signs called 'Jyūnishi[十二支（じゅうにし）](literally, 12 branches)'.
Jikkan consists of following 10signs: 甲(Kō), 乙(Otsu), 丙(Hei), 丁(Tei), 戊(Bo), 己(Ki), 庚(Kō), 辛(Shin), 壬(Jin), 癸(Ki)
Jyūnishi consists of following 12 signs(Chinese reading / Japanese reading): 子(Shi / Ne), 丑(Chū / Ushi), 寅(In / Tora), 卯(Bō / U), 辰(Shin / Tatsu), 巳(Shi / Mi), 午(Go / Uma), 未(Bi / Hitsuji), 申(Shin / Saru), 酉(Yū / Tori), 戌(Jyutsu / Inu), 亥(Gai / I)
The combination goes around in 60. For example, the system works as following:
1. 甲子 → 2. 乙丑 → 3. 丙寅 → 4. 丁卯 → 5. 戊辰 → 6. 己巳 → 7. 庚午 → 8. 辛未 → 9. 壬申 → 10. 癸酉 → 11. 甲戌 → 12. 乙亥 → 13. 丙子 → 14. 丁丑 ......... 58. 辛酉 → 59. 壬戌 → 60. 癸亥 → 1. 甲子
12 animals as symbols are allocated for Jyūnishi but its reason is uncertain. And animals are different from country to country. In Japan, they are as follows: 子(Rat), 丑(Ox), 寅(Tiger), 卯(Rabbit), 辰(Dragon), 巳(Snake), 午(Horse), 未(Sheep), 申(Monkey), 酉(Cock)・戌(Dog)・亥(Boar)
Eto is no longer used except for representing year that is expressed by animals in many countries. It applies to Japan too.
In Japan, when '干支' is pronounced as 'Eto', that also refers to only Jyūnishi. Originally, that was not correct because the word '干支' derives from '十干' and '十二支'. But now, we recall Jyūnishi when we hear 'Eto' because Jikkan is no used except for astrology and religion.
The year by Eto is also not used in daily life. We only recognize the animal of the year, especially that of new year, and that of the year when we were born.
And that is often used as the design of Nenga-jyō[年賀状（ねんがじょう）] which is a greeting card for Shōgatsu[正月（しょうがつ）](Japanese New Year).
When the year of same animal, which a person was born in, comes(when the person is 12, 24, 36, 48...years old), the person is called 'Toshiotoko[年男（としおとこ）](man of the year)' or 'Toshionna[年女（としおんな）](woman of the year)'.
And reaching 60 years old is called 'Kanreki[還暦（かんれき）]'. '還暦' literally means a cycle of Eto completes and returns to the start point.
There is a custom of sending red clothes to someone who reaches Kanreki. Because red was believed to have an effect to expel evil and people put red clothes on babies. Therefore, it means that someone who reaches Kanreki goes back to the time when it was born and it is reborn due to go round a cycle of Eto.
Animal of Eto of 2016 is Monkey and that of 2017 is Cock.