Nested loops joins are the join operator you are likely to see the most often. It tends to operate best on smaller data sets, especially when the first of the two tables being joined has a small data set.
In row mode, the first table returns rows one at a time to the join operator. The join operator then performs a seek\scan against the second table for each row passed in from the first table. It searches that table based on the data provided by the first table, and the columns defined in our ON or WHERE clauses.
Read on for more information about nested loop joins.