Common visual conventions seen in manga and anime to convey emotions, often for comical effect.
Over the years mangaka have developed a visual language within manga to convey emotions in a very exaggerated way. As with most manga conventions, they have also migrated into anime. Sweat Drops, Popping Veins and so on all generally convey different emotions, but may appear on characters at the same time, to express both sadness and anger for example.
The Sweat Drop is a popular icon that can be used to express nervousness, worry, embarrassment or confusion.
Blushing is an exaggeration of real-life blushing, blushing can be used to convey embarrassment, arousal, happiness or to show a character is drunk or even ill.
The Popped Vein is usually red or black in color, and occasionally blue or purple, popped or "cross-popping" veins are shown to convey anger or frustration.
Gloom Lines are usually a dark blue or purple color, parallel lines appear on the face, above the head or in some cases even behind characters as a large backdrop. Straight lines may represent mortification, fatigue, or even horror, while wavy lines may represent depression or disgust.
The Visible Sigh is a sign of exasperation, a small cloud often resembling a mushroom appears from their mouth, sometimes accompanied with a sound effect.