A long barrow is a roughly rectangular or trapezoidal mound of earth and/or stone, usually between 25m and 120m long, with a length exceeding twice its greatest width. The mound may be edged with a timber or stone revetment, and there is frequently a facade at the higher and/or wider end of the monument. The majority of long barrows contain one or more stone or wooden burial chambers which occupy a small portion of the total structure. A few long barrows are known to lack burial chambers. Other distinctive components include quarry pits, flanking quarry ditches, and a forecourt.

There is considerable regional variation within the class as a whole, many of the superficial differences being caused by the use of locally available building materials.

Long barrows are usually recognized as earthworks, but can sometimes be identified from aerial photographs. Possible sources of confusion can arise when differentiating long barrows from oval barrows, D-shaped barrows, long mortuary enclosures, portal dolmens, simple passage graves and various other classes of megalithic tomb. Confusion can also arise with pillow mounds, waste heaps connected with quarrying, isolated sections of bank, natural rises in the ground, and small moraine-hills. In all cases, close attention to shape, size, overall form, visible components, location, and associations are important when distinguishing long barrows.

The term long barrow is here taken to include monuments with barrow mounds of earth (sometimes called earthen long barrows) and those with barrow mounds of stone (sometimes called long cairns). Specifically excluded from the class of long barrow are bank barrows, which are generally over 180m in length, and long mounds which are very long and narrow and lack many of the key components found in long barrows.

Long barrows appear to have served as burial places and ritual foci for communities living in the immediate neighbourhood. As such, their function may have overlapped with that of other broadly contemporary classes of monument, for example oval barrows, D-shaped barrows, bowl barrows, and long mortuary enclosures.