The one-sentence difference
If the ice is still part of a land-based flow, you are looking at a glacier (or an ice sheet, which NSIDC treats as glacial ice covering more than 50,000 square kilometers). If a chunk has already broken off and is floating, you are looking at an iceberg.
NSIDC's glacier overview states the mechanism directly: at higher elevations more snow falls than melts, the surplus ice begins to flow downhill, and at lower elevations melt or iceberg break-off removes mass. Calving is how glaciers feed icebergs; the berg is the product, not another name for the glacier.
Glaciers and ice sheets
NSIDC describes alpine glaciers as frozen rivers of ice flowing down mountainsides into valleys, and notes that glaciers also exist on the fringes of ice sheets. An ice sheet, in NSIDC's definition, is glacial ice on land that extends more than 50,000 square kilometers (about 19,300 square miles).
Earth currently has two ice sheets, Antarctic and Greenland. NSIDC says together they hold more than 99 percent of the freshwater ice on Earth. During the Last Glacial Maximum about 20,000 years ago, the Laurentide Ice Sheet covered much of North America; NSIDC notes that its weight helped create the basins that now hold the Great Lakes.
Those land-based systems can shed icebergs where they meet the ocean, but the parent ice remains a glacier or ice sheet until the break happens.
What makes an iceberg
NOAA's Ocean Service fact page states the definition in one line: an iceberg is ice that broke off from glaciers or shelf ice and is floating in open water. The break-off step is required. A grounded ice tongue still connected to land is not yet an iceberg under that wording.
Because bergs begin as glacial or shelf ice, they are freshwater ice even when they float in salt water. Size varies from growlers and bergy bits too small for ship charts up to tabular bergs the size of cities; the definition does not change with size, only the hazard does.
People sometimes call any floating ice an iceberg. Sea ice, which freezes from ocean water, is a different category. This page sticks to NOAA's break-off definition.
A quick field test
Ask whether the ice is still flowing as part of a land ice mass. If yes, call it a glacier (or ice sheet). If you are watching a detached block drift after a break-off, call it an iceberg.
For other cryosphere lookalikes on this site, keep the comparison inside ice and climate topics rather than weather vortices. The useful neighbor pages are glacier and iceberg definition pages themselves plus any ice-sheet explainers you already trust from NSIDC.
For other Earth Science contrasts on FactCrumbs, see Ganymede vs the Moon and is ball lightning dangerous.
Calving: how glacier ice becomes bergs
Calving is the mechanical break-off of ice from a glacier terminus or an ice-shelf front into water. NSIDC treats calving as one of the ways land ice loses mass to the ocean, alongside surface melt and basal melt under floating shelves. The parent ice is still a glacier or shelf until the fracture finishes; the floating piece is then an iceberg.
Not every calving event looks the same. A grounded tidewater glacier can shed irregular blocks as the front oversteepens. A floating ice shelf can lose long, flat slabs when rifts open parallel to the front. In both cases the process is fracture plus buoyancy, not a rename of sea ice that froze from seawater.
After calving, currents and wind take over. The berg may roll, break into smaller pieces, or ground on a shallow bank. Size class names used by ice observers (growler, bergy bit, small/medium/large/very large berg) describe the floating fragment, not the glacier that produced it.
Tabular vs irregular bergs, and ship-scale classes
Tabular icebergs have steep sides and a flat top. They often come from ice shelves, where the parent ice was already floating as a thick slab. Irregular or non-tabular bergs show pinnacles, slopes, and uneven silhouettes more typical of glacier-front calving or of tabular bergs that have rolled and fractured.
Operational ice reports use rough size bins for navigation. Growlers are small pieces that can still damage a hull but sit low in the water. Bergy bits are larger than growlers yet well below a full berg. Above that, observers step through small, medium, large, and very large classes by waterline length or height. The exact bin edges vary by reporting office, so treat the labels as hazard classes, not species names.
A key navigation point follows from density: most of a berg's volume sits below the waterline. What you see above water is a minority of the ice. That is why charts and ice bulletins care about drift tracks, not postcard silhouettes.
Ice shelves vs glacier termini
An ice shelf is the floating extension of a glacier or ice sheet that has flowed off the land and spread over the ocean while remaining attached. NSIDC distinguishes shelves from free-drifting bergs: the shelf is still part of the land-ice system until a rift frees a slab. Greenland and Antarctica host the large modern shelves; alpine glaciers more often calve from a grounded or partly floating terminus without a continent-scale shelf.
Iceberg production at a shelf front can create the long rectangular bergs that make news when they are tens of kilometers across. Tidewater glacier fronts more often shed smaller, blockier pieces. Both products are freshwater glacial ice once detached. Neither should be confused with sea ice, which forms when seawater freezes and can be salty or brine-rich depending on age.
When you read a headline about a "new iceberg the size of X," check whether the parent was a named ice shelf or a glacier outlet. That parentage tells you whether the event was a shelf-front calving or a terminus break, and it keeps the glacier-versus-iceberg vocabulary honest.
What this distinction is not
Glacier versus iceberg is not the same contrast as glacier versus ice sheet. An ice sheet is glacial ice covering more than about 50,000 square kilometers in NSIDC's common teaching definition; today that means Greenland and Antarctica. Icebergs can calve from both alpine glaciers and ice-sheet outlets.
It is also not a salt-versus-fresh label for everything floating. Bergs are freshwater ice in seawater. Sea ice is frozen seawater. Pack ice and floes belong to the sea-ice conversation unless a report explicitly says glacial bergs are mixed in.
Finally, meltwater lakes on a glacier surface are not icebergs. They sit on land ice. Only after ice detaches and floats freely does the iceberg label apply.