1. Kilo: Metric unit representing 1000. E.g.: 1kHz is a 1 kilohertz (1000 Hertz). Note that the k is always lowercase.
In digital systems, "K" or "k" is often used to mean 210, that is, 1024. This is not well-standardized but it's usually apparent from context. On the Maxim site, we use upper-case K to mean 1024 and lower-case k to mean 1000. This standard is applied to new documents but older documents may use "k".
2. Kelvin: Temperature scale. Zero K is defined as absolute zero. 273.15K is 0 degrees C.
Note that temperatures on the kelvin scale are called kelvins, not "degrees kelvin." The K symbol is uppercase and used without a degree symbol. The word "kelvin" in this context is not capitalized.