Landwalk is an evergreen ability, usually specialized to one of the five basic lands such as "Islandwalk" or "Swampwalk." Most interestingly "Plainswalk" is quite rare as it is obviously most difficult thematically. Walking through an open field is akin to Plainswalk, while swimming underwater is akin to Islandwalk.

702.12. Landwalk

• 702.12a. Landwalk is a generic term that appears within an object's rules text as "[type]walk," where [type] is usually a subtype, but can be the card type land, any land type, any supertype, or any combination thereof.
• 702.12b. Landwalk is an evasion ability.
• 702.12c. A creature with landwalk is unblockable as long as the defending player controls at least one land with the specified subtype (as in "islandwalk"), with the specified supertype (as in "legendary landwalk"), without the specified supertype (as in "nonbasic landwalk"), or with both the specified supertype and the specified subtype (as in "snow swampwalk"). (See rule 509, "Declare Blockers Step.")
• 702.12d. Landwalk abilities don't "cancel" one another.

  Example: If a player controls a snow Forest, that player can't block an attacking creature with snow forestwalk even if he or she also controls a creature with snow forestwalk.

• 702.12e. Multiple instances of the same kind of landwalk on the same creature are redundant.

Translation:

• 702.12a. - Landwalk provides the rules for the specialized type of land walking.
• 702.12b. - This rule pertains to how blocking can be declared, like Flying and Shadow.
• 702.12c. - Defending player can't block the attacking creature if he (the defender) controls the defined type of land.
• 702.12d. - Landwalk does not allow blocking of landwalking creatures.
• 702.12e. - A creature with more than one instance of landwalk is not "more" unblockable.