About this character
U+2603 SNOWMAN was added in Unicode 1.1, published in June 1993, as part of the original allocation of the Miscellaneous Symbols block (U+2600–U+26FF). It predates emoji as we now think of them by nearly two decades: when the codepoint was assigned, the symbol was intended as a weather glyph alongside ☀ SUN, ☁ CLOUD, and ☂ UMBRELLA, modelled on the symbols used in printed weather forecasts. Its default presentation in the Unicode standard is text, not emoji — which is why, with most fonts, U+2603 renders as a calm, single-coloured line drawing rather than the rounded, smiling 3D figure most people picture.
The character that does render as the colourful emoji on phones is U+26C4 ⛄ SNOWMAN WITHOUT SNOW, added later and given an emoji-presentation default in Unicode 6.0 (2010). The two are constantly confused. If you paste ⛄ into a text editor with the default monochrome font you will likely see an empty snowman; paste ☃ and you get a snowman with three flecks of snow above it. To force U+2603 into its emoji presentation, follow it with U+FE0F (the emoji variation selector): ☃️. To force text presentation, follow with U+FE0E.
Programmers sometimes use U+2603 as a placeholder or test character precisely because it is unambiguous, prints as a single visible glyph, and exercises the three-byte UTF-8 path. It also appears in the UTF-8 encoding examples in this site's guides — E2 98 83 is a useful sequence to remember when debugging mojibake. Its decimal value, 9731, is the entity reference you will see in older HTML escaped output: ☃.
From the Unicode data
Its emoji name (CLDR short name) is “snowman”, in the Travel & Places group, subgroup sky & weather. By default it is shown as a text glyph; follow it with U+FE0F VARIATION SELECTOR-16 to request the colour emoji form (U+2603 U+FE0F), or with U+FE0E VARIATION SELECTOR-15 to force text style.