Global Illumination: It is a method (algorithm) of computation for light calculation in the scene which takes into account the light bounces from the neighboring surfaces, along with the normal illumination of direct lights. In other words GI calculates the Indirect light also, thus it makes the renders more photo-realistic. Examples of GI methods are Radiosity and Ambient Occlusion in Blender and on a general scale Radiosity, Ray tracing and Caustics all use different GI algorithms.
Ray tracing: A method in CG which uses an algorithm to calculate the effect of lights on the surface of objects in the scene. In CG the ray-tracer works by calculating the light effect on the scene by tracing the light photons back to the point of origin, from the scene or the camera. It uses the reverse of what is in real life, the sun shoots photons and we receive them through reflection/refraction from the objects, the photon energy is also modified by the objects by absorption or adsorption to form a particular texture or colour of the object.
The reverse way of ray-tracing is done so as to reduce the amount of calculations, as it is faster to take just the photons or lights which reach the scene or our eye or the camera, than calculating everything what is outside the view.
Photon: The photon is also referred as Ray of light. And it is the smallest unit of light energy.
Caustics: The caustics are referred to as the refraction pattern formed by highly transparent objects such as a glass of fluids which have a certain degree of total internal reflection, for example the light falling on a glass filled with wine will form some strange patterns of different colours and intensity which are referred to as light caustic. The computer method for Caustic calculation is also referred to as photon mapping. The photon lamp in Blender/Yafray is for this purpose only.
HDRI: Or High Dynamic Range Illumination. This method is relatively new in GI. This uses the actual light probe value of a real scene in the real world taken through with special equipment to produce a 360° view of a scene, and stores the information of the light from all areas in the scene in a spherically mapped image called HDR. The renderer uses that information to shoot light from and provides even more photo-realistic rendering.
Anti-aliasing: Refers to a method to reduce the brightness levels between two neighboring pixels by overlapping the colours to average the level between neighboring pixels. This make the images appear smoother.
Photonmap: An assumed area or bound with-in which more density of photons are kept for calculations of indirect illumination.