It's coming on winter, at least in the northern hemisphere, where Italy is located.

In many places in Italy, people heat their houses using wood. Or, In the country and in small villages, lots of people have fireplaces in their kitchens. 

BANNER PLACEHOLDER

Wood is Wood, right?

Right and wrong. In English, we think of wood as wood, whatever its use. But in Italian, there are two similar but different words, depending on what we do with the wood.

Legno

To construct something we use legno (wood), a masculine noun. This has its root in the Latin noun "lignum." 

Interestingly, Italians use two basic prepositions with legno to correspond to "wooden": in and di which can both mean "of."

Questo meraviglioso piano in legno

This marvelous wooden surface

si chiama spianatoia

is called a pastry board

e serve proprio per impastare la nostra pasta fresca.

and it's used exactly for making our fresh pasta dough.

Captions 90-92, L'Italia a tavola - Culurgiones D'Ogliastra

 Play Caption

Veniva impastato in casa,

The dough was worked at home,

proprio su quella superficie di legno

right on that wooden surface

e poi messa [sic: messo], questo impasto,

and then this dough was put

su quella specie di tavola, veniva portato al forno,

on that type of wooden board and brought to the oven,

perché in casa non c'erano dei forni.

because there were no ovens in houses.

Captions 64-68, Meraviglie - EP. 1 Part 12

 Play Caption

Legna

To build a fire for heating or cooking, we use the feminine noun la legna. This comes, again from the Latin, from the plural of "lignum": "ligna." In fact, la legna, just like the collective noun "firewood," usually refers to a collection of pieces of wood to be used for burning. 

If we ask what kind of wood is used, then we can use legno. In the following example, someone is asking the pizzaiolo what kind of wood he uses in his forno a legna.

Quello è il forno a legna. Che legno usate?

That's the wood oven. What kind of wood do you use?

Captions 39-40, Antonio - presenta la Pizzeria Escopocodisera

 Play Caption

To be even more specific, we can expand on legna: legna da ardere (wood for burning/firewood). The following example is from a fascinating video on Yabla about olive trees and making olive oil.

Quando avveniva questo distacco delle due parti dell'ulivo,

When this detachment took place of the two parts of the olive tree,

una della due parti veniva sacrificata come legna da ardere.

one of the two parts was sacrificed as firewood.

Captions 47-48, Olio Extra Vergine Pugliese - Introduzione e cenni storici

 Play Caption

What are some contexts for legna?

The fireplace is often called il camino (note the single M) and more often than not, the diminutive is used:  il caminetto. The chimney is the canna fumaria (the smokestack). 

In place of la  caldaia (furnace, hot water heater), some people have una stufa a legna (wood stove).

And let's not forget that the best pizza is said to be made in a forno a legna (wood-burning oven). In these cases the preposition a is used, referring to the function. What makes it run?

Peppe ha infornato la pizza nel forno a legna, che è un forno tradizionale.

Peppe has put the pizza in the wood oven, which is a traditional oven.

Caption 48, Antonio - presenta la Pizzeria Escopocodisera

 Play Caption

This goes for bread, too.

Antico a lievitazione naturale, cotto a legna, ci sono altri tipi...

Traditional sourdough, baked in a wood oven, there are other kinds...

Caption 64, Anna e Marika - Il pane

 Play Caption

BANNER PLACEHOLDER

Now you know the difference between legno and legna. They are both right; you just need to know the context.