This is a more technical description for those who are interested in
Esperanto from a linguistic point of view.

Phonemically Esperanto's repertoire consists of 23 consonants and 5
vowels. All of these phonemes are present in Scots and Scottish
dialects of English (although not all of them are in English and
American dialects of English). Segment length is not phonemic.

Stress is regularly placed on the second last syllable of each
polysyllabic word, adjacent vowels counting as separate syllables.
Sometimes stress occurs elsewhere for the purposes of emphasising a
particular morpheme of a word to make the meaning clearer. Secondary
stresses may also be present in longer words. Stress is indicated by
volume rather than pitch.

Declarative and interrogative sentences tend to have different
intonation patterns from one another, as in most European languages.
Different tones are not significant at a segmental level.

Typologically, Esperanto is mostly an agglutinating language, like
e.g. Swahili, Turkish or Quechua, in that each affix has a fixed
meaning. However, as each affix can also be used independently as a
root and sometimes even as a word, this means that the language also has an
isolating nature to it as well. The number of bound morphemes and
other features typical of a flexional language are severely limited in
Esperanto.

Esperanto is an accusative language, explicitly marking objects by a
case ending where possible. Standard word order is SVO, but the
ability to mark the majority of objects allows a substantial degree
of freedom in this respect and OSV is the next most common sequence.
Esperanto is prepositional, and attributive adjectives, numerals and
determiners normally precede their noun. In word formation, modifiers
normally precede their head. Clause structure is oriented around the
verb in Esperanto. Syntax in Esperanto exhibits considerable freedom.
The positioning of adjuncts is less rigid compared with English and
German, for example.

There is no grammatical gender or noun class in Esperanto. Plurality
and definiteness (but not usually indefiniteness) are marked in noun
phrases. There is agreement in case and number for adjectives. Verbs
possess features for indicating tense (present, future and past),
voice (active, passive, medial and causative) and mood (indicative,
conditional and volitive). Some features for marking aspect are also
available, but optional. Verbs do not indicate person or number.

Esperanto has a well-developed apparatus of derivational morphology.
This leads to a useful level of lexical economy which is reminiscent
of Creoles, but is much more systematic and developed. Literary and
scientific registers of the language supply a countervailing tendency
to this. Esperanto has acquired a number of phraseological features
which are reminiscent of the results of creolisation.

Lexically, the majority of items are derived from (one hesitates to
use the phrase 'cognate with') etyma in Romance languages and
classical Latin, but there are also Germanic, Greek and Balto-Slavic
elements. In some cases changes have been made to the form of certain
roots to avoid clashes with derivational morphemes, or to resemble the
written form in the source language rather than the spoken one, or
simply to make the root shorter. There are some items that cannot be
allocated to an obvious etymology, but which are all distinctly
Indo-European. Many derived words, however, are internal to Esperanto
in that their equivalents in the source languages are not derived.

Esperanto avoids, but does not prohibit, polysemy. There are a number
of paronyms that have been created by slight changes to the etymon to
distinguish them. It is possible, however, to create some isomeric
forms by derivational morphology which can be analysed in more than
one way. This is a source of humour in Esperanto analogous to puns in
languages which have more developed polysemy. New words are created
in Esperanto in the same way as any other living language, borrowing
externally or creating from internal resources.

There is evidence of some diachronic change, mostly but not entirely
lexical, although the pace of this has slackened in recent decades.
Evidence of synchronic variation has always been quite small.

Esperanto is, of course, a planned language. It is overwhelmingly
a posteriori, but has a number of schematic features. It is currently
the only planned language with a developed worldwide speech community.
(Only Ido and Volapuk have also achieved this, but to a lesser degree
and in the distant past, and they are now on the verge of extinction).
There are probably about 50000 fluent active users of Esperanto at any
given time, plus a much larger population of learners and former
learners. There are also several hundred native speakers of Esperanto,
all of whom are bilingual or multilingual with other languages.


Further reading:

Most literature which discusses Esperanto from a linguistic point of
view is in Esperanto, including notably:

Wells, John C.: "Lingvistikaj aspektoj de esperanto", Rotterdam: UEA,
1978.
Kalocsay, Kalman & Waringhien, Gaston: "Plena analiza gramatiko de
esperanto" (5th edition), Rotterdam: UEA, 1985
Waringhien, Gaston: "Lingvo kaj vivo: Esperantologiaj eseoj",
Rotterdam: UEA, 1989

By far the best work available in English is:

Gledhill, Christopher: "The Grammar of Esperanto: A corpus-based
description" (2nd edition), M?chen: Lincom, 2000

Those interested in Esperanto from a sociological perspective rather
than a linguistic one may also find the following work interesting,
although the information in it is becoming rather dated.

Forster, Peter G.: "The Esperanto Movement", Den Haag: Mouton, 1982