seq2seq an Encoder-Decoder structure network, its input is a sequence, the output is a sequence, a signal sequence will Encoder variable length becomes a fixed-length expression vector, this vector Decoder fixed length may become the sequence of the target signal variable length.
The structure of the most important place in that the input sequence and an output sequence length is variable, may be used for translation, bot, syntactic analysis, text summaries.
encoder-decoder structure: https://arxiv.org/pdf/1406.1078.pdf
seq2seq structure: https://arxiv.org/pdf/1409.3215.pdf
reference: