What is the difference between internal and external validity?

Internal and external validity both relate to the evaluation of a research's study / procedure' validity.
Internal validity refers to whether there is a causal relationship between the independent variable (IV) and the dependent variable (DV), i.e. whether the observed effects on the DV are the direct result of manipulating the IV, and not some other factor that has not been accounted for. For example, to ensure that internal validity is attained the researcher can start by controlling for extraneous variables, using standardized instructions, counter-balancing, and eliminating demand characteristics and investigator effects (these terms are to be explained additionally if tutee shows lack of understanding/requests it).
In contrast, external validity describes the extent to which the study's results are generalisable over time (historical validity) and to other contexts/settings(ecological validity) and people (population validity). There are multiple ways for improving the validity of this construct; some examples include using more natural settings for one's experiment, or selecting for participants using random sampling.