Nothing - they are the same. External beam means primarily that the radiation source comes from outside (in contrast with brachytherapy which is "internal") such as a LINAC which delivers ionizing radiation (AKA Linear Accelerator).

IMRT stands for Intensity Modulated Radiation Therapy. It was introduced about 1994. It is a device that attaches to the primary collimator (beam forming device) of the LINAC and allows the beam to be broken into pencil sized beamlets (with a device called a MLC or Multi-Leaf Collimator) and these beamlets can be shaped and formed to conform to the actual tumor shape itself. It is even dynamic in that the beam can be continuously shaped during the treatment. An earlier form of this was called 3D conformal.

The advantage of IMRT is that there is less healthy tissue damage. Beam intensities can be moderated through radiation sensitive tissue areas. What this translates to is that often some salivary function can be spared. They can also save the thyroid in many cases.

An even earlier type of external beam radiation was XRT. It was developed in the 60's to replace Cobalt 60 machines which which could cause irreparable skin damage as the radioactive source aged and longer exposure times were required. The beam shaping is much less precise so a larger area is radiated as a result. In poorly differentiated cancers this may be the treatment of choice.

The very first type of external radiation was done with Cobalt 60 and the first successful treatment with external radiation for a head & neck cancer was in the late 1800's at the Curie Institue in Paris. Some foreign countries still use Cobalt 60 machines. It has been virtually eliminated as a treatment of choice in the US and replaced by the LINAC.

Proton Beam therapy was under development beginning in the early 90's, which uses a charged particle instead of ionizing radiation is the latest development in external beam radiation treatment modalities.