In Germany, and I imagine it’s not very much different in most other countries with a parliamentary system:
The larger parties have
[ul]
[li]a party program (a document of essential values and policies) that has a lifetime of several parliamentary terms. For example my party, the SPD, voted party programs in 1869, 1875, 1891, 1921, 1925, 1959, 1989 and 2007. The party program is less an appeal to voters than a document that is the occasion for the party membership to readjust the party’s general ideological stance.[/li][li]an election program, on each level of government (federal, state, district, municipiality), voted on before the start of the election campaign for the next term. These election programs are prospectuses of what the party intends to do on the respective level of government during the next term.[/li][/ul]
These programs are debated and voted on
[ul]
[li]on the federal and state level, and partly on the district level: by a conference of delegates elected from the membership (possibly indirectly, by state conferences of delegates)[/li][li]on the local level: directly by the membership[/li][/ul]
The requirements for joining a political party are: to fulfil the requirements of the party’s bylaws. Most parties’ only requirement are a minimum age (often 16 years), some require a length of residency in Germany or even citizenship.
The local party committee often has a right of veto over new memberships (for people that are locally known, and not in a good way).
Parties can expel members automatically if they are a member of, or run for office for, a competing party, or if they are a member of a nonparty organization considered incompatible. Parties also expel members for behaviour considering odious and bringing the party into disrepute. Expelled members can but usually do not fight their expulsion in court.
The main obligation for a party member are to pay their dues, either on a scale by income or a flat due (whether they have fulfilled that obligation can become an issue at candidate nomination time). For example I pay membership dues of 40 €/about 45 USD per month. The main right of party members are to speak and vote in local party meetings, and to be elected to party office.
Independent candidates elected: very, very rare on the federal and state level, a minority in municipial councils, very prevalent as majoral candidates in those states where mayors are elected directly as opposed by the town council.