PAL (Phase Alternating Line) refers to a method of adding the colour difference information to the luminance information in a Composite Video signal.
When television was invented, it was Black & White (and the programs were better, but that's hardly relevant here...). Hence only a luminance signal was transmitted. When colour came along, broadcasters wanted to upgrade in a way that meant existing monochrome receivers could still display a picture. They did this by broadcasting the additional colour difference information that colour receivers would require, on a subcarrier frequency (round about 4.43MHz) added to the main signal. Existing monochrome receivers would ignore this additional information.
Everything in the paragraph above describes the NTSC television standard too. However, PAL goes one step further. Due to weird transmission, weather and atmospheric phenomena, the phase of the colour difference signal can shift between the transmitter and the receiver, resulting in the receiver displaying the wrong colour. NTSC receivers often have a manual Hue control for the viewer to correct this. However, with PAL, the phase of the colour difference signal is inverted on alternate lines (hence it's name Phase Alternating Line).
The shift in the colour with PAL is thus opposite on alternate lines at the receiver. Older receivers relied on the human eye to average out succesive lines to perceive the correct colour, however newer receivers include additional electronics to do this so that the correct colour is displayed on all lines.
PAL-I is the UK television standard. The I refers to the frequency of the sound subcarrier (among other technical details).