Short answers, pulled from the story.
The term ecchi originated in 1952 when the magazine Shukan Asahi reported a woman groped by a stranger in a movie theater who reacted with the phrase hey, that's perverse. This event marked the beginning of the abbreviation of hentai evolving from clinical terminology to slang for sexual forwardness. The word later became the standard term for abnormal sexual desire in the 1910s before the Second World War.
The word ecchi describes manga with very light or playful erotic content such as is found in shonen manga, whereas hentai connotes perversion or fetishism in Western culture. If a young woman were to call a young man ecchi, that might be construed as flirting, whereas the full word sounds more like condemnation. The Japanese media tend to use other words, such as ero-manga for adult manga or anime for persons over 18 years.
Common elements include conversations with sexual references or misunderstandings, such as double entendre or innuendo, which serve as the narrative engine for these stories. Visual depictions often involve suggestive posing, revealing or sexualized clothing, and nudity, often involving clothing malfunctions where garments are ripped apart or become wet. Graphically, techniques are used to show sexy pictures by revealing parts of the female body such as the back or breasts.
The use of panty shots, or visibility of the underwear, is one common motif that serves as a visual shorthand for character personality and sexual experience. Typically, the male character will react in an exaggerated manner and be castigated, while the color and style of the panties are an indication of the character, personality, and range sexual experience of the female character. Panties are the main theme in some ecchi works, such as Chobits and Panty & Stocking with Garterbelt.
Levels of nudity vary strongly between works, depending on the intended audience and the preferences of the authors, leading to a complex dance of censorship that varies widely across television stations. In some cases, though the breasts are shown on the screen, nipples and genitals are obscured by props, clothing, or effects, a kind of censorship that was typical for characters like Lala Satalin Deviluke in To Love Ru. Conversely, in Ladies versus Butlers! and other such anime, the nipples are clearly visible through clothing, no matter how thick it is.
The concept of ecchi is very closely related to fan service, yet while fan service describes every aspect to please the fans, ecchi relates specifically to sexual themes. A special kind of fan service, that is usually bound or justified by the narrative, ensures that these sexual elements are not merely gratuitous but serve a purpose within the story. Ecchi themes are a type of fan service, and can be found in most comedy shonen and seinen manga and harem anime.