Reference: https://www.tensorflow.org/get_started/get_started

1. Multiple level APIs:

(1) lowest level: TensorFlow Core, if you need fine controls on the model

(2) higher levels: built on Core, easier to use, e.g. *tf.contrib.learn*

2. **Tensor**: an array of primitive values. **Rank**: how many dimensions. **Shape**: a vector, containing the numbers of elements in each dimension.

e.g. [[[1., 2., 3.]], [[7., 8., 9.]]] has rank 3, shape [2,1,3]

3. Import library

import tensorflow as tf