Seek requires an index on the table and that the index be active (set order to). It searches for the expression you specify in the index, and selects the appropriate record if a match is found. It is always very fast.

Locate looks through the table for a reccord that matches the conditions you specify. You can feed it just about any matching conditions, multiple fields, calculations etc... When there are indexes on the table that it can use for the search, it works quite fast, but if there are not it takes much longer than a seek.