Theano is an open source Python library for building neural networks.
In its most basic form, Theano is an optimizing compiler for mathematical expressions. It is specially designed to handle the computation required for large neural network algorithms used in Deep Learning.
Theano is written in Python & works well with Python libraries such as NumPy.
It was created at MILA (Montreal Institute for Learning Algorithms) at the University of Montreal. Development on the project started in 2007 and Theano has been around for over a decade. It is a pioneer in the field of Deep Learning research and development. It is the tool behind many research breakthroughs.
Joshua Bowen's Notes 