What kind of ants are these? Are these venomous?
These ants are called weaver ants, they are in the genus Oecophylla. 

'Helcium is attached relatively high on abdominal segment III only in Oecophylla'
http://www.antwiki.org/wiki/Formicinae

They use the silk produced by their larvae to bind leaves together to make chambers for their brood. As any ants in the Formicinae subfamily, they do not have a stinger. They can however bite and drop formic acid on your skin, which can cause irritation. They are used in orchard as biological control agents against pests.