Quil-T is an extension to Quil which introduces pulse-level control to quantum programs. With Quil-T one can describe a program at a level lower than is typically permitted in circuit-type programs, with explicit control over the RF waveforms played by the QPU’s control hardware. In particular this imbues programs with a notion of time, hence the T suffix.
The Quil compiler quilc was developed to support most users in their pursuit for producing an optimal program from a high-level language. In contrast Quil-T was developed to enable the low-level and precise control desired by power-users. For example, for many users the implementation details of a Hadamard gate are not particularly important, and indeed the behind-the-scenes realisation of a Hadamard gate are likely to change over time as gate implementations are recalibrated to provide the best results. If you instead you are interested in those details, and in particular you want to control those details, then pulse-level control with Quil-T is the way to go. With Quil-T you can define precisely what you mean by H 0, you can perform experiments to characterize the underlying hardware such as determining T1. The hardware is almost at your fingertips.
For examples, see the adjacent notebooks. For more information, see the Quil project homepage.